Update the "Adding Field Type" howto.

Closes #48.

content/docs/api/db/type/contents.lr

@@ -41,3 +41,6 @@ def setup_env(self, **extra):
 ```
 
 For more information see [value_from_raw :ref](value-from-raw/).
+
+There is a more complete example in the
+[Plugin How To :ref](../../../plugins/howto/#adding-new-field-types).

content/docs/plugins/howto/contents.lr

@@ -114,64 +114,76 @@ function we also track the plugin's filename to rebuild if the plugin changes.
 
 ## Adding New Field Types
 
-Let's say you want to add a "datetime" type to the set of
-[field types :ref](../../api/db/types/) used in your models, because the
-built-in "date" type has no time component. You might update `blog-post.ini`
-from the [blog guide :ref](../../guides/blog) like so:
+Let's say you want to add an "asciidoc"
+[field type :ref](../../api/db/types/) so you can write with [AsciiDoc](http://www.methods.co.nz/asciidoc) markup.
+
+First [install AsciiDoc](http://www.methods.co.nz/asciidoc/INSTALL.html) so its command-line program is available. Then update `blog-post.ini` from the [blog guide :ref](../../guides/blog) like so:
 
 ```ini
-[fields.pub_date]
-label = Publication date
-type = datetime  # Custom "datetime" type.
+[fields.body]
+label = Body
+type = asciidoc  # Custom type.
 ```
 
-In a blog post's `contents.lr`, specify its publication date like:
+In a blog post's `contents.lr`, write some AsciiDoc like:
 
 ```
-pub_date: 2016-01-02 12:34:56
+body:
+
+== Header 1
+
+Some text.
+
+-----
+code here
+-----
 ```
 
-You can add your "datetime" type to Lektor with a plugin:
+You can add your "asciidoc" type to Lektor with a plugin:
 
 ```python
-from datetime import datetime
+from subprocess import PIPE, Popen
+
 from lektor.pluginsystem import Plugin
 from lektor.types import Type
 
-class DatetimeType(Type):
+
+def asciidoc_to_html(text):
+    # The "-" at the end tells asciidoc to read from stdin.
+    p = Popen(
+        ['asciidoc', '--no-header-footer', 
+         '--backend=html5', '-'],
+        stdin=PIPE, stdout=PIPE, stderr=PIPE)
+
+    out, err = p.communicate(text)
+    if p.returncode != 0:
+        raise RuntimeError('asciidoc: "%s"' % err)
+
+    return out
+
+
+# Wrapper with an __html__ method prevents
+# Lektor from escaping HTML tags.
+class HTML(object):
+    def __init__(self, html):
+        self.html = html
+
+    def __html__(self):
+        return self.html
+
+
+class AsciiDocType(Type):
+    widget = 'multiline-text'
+
     def value_from_raw(self, raw):
-        return datetime.strptime(raw.value, '%Y-%m-%d %H:%M:%S')
+        return HTML(asciidoc_to_html(raw.value or u''))
 
-def strftime_filter(dt, fmt='%B %-d, %Y'):
-    return dt.strftime(fmt)
 
-class DatetimeTypePlugin(Plugin):
-    name = u'datetime type'
-    description = u'Adds a new field type, "datetime".'
+class AsciiDocPlugin(Plugin):
+    name = u'AsciiDoc'
+    description = u'Adds AsciiDoc field type to Lektor.'
 
     def on_setup_env(self, **extra):
-        self.env.types['datetime'] = DatetimeType
-        self.env.jinja_env.filters['strftime'] = strftime_filter
-```
-
-This plugin also adds a `strftime` filter to Jinja.
-Render a datetime with the default format in a template like:
-
-```html+jinja
-Published on {{ this.pub_date|strftime }}
-```
-
-Or a custom format:
-
-```html+jinja
-Published on {{ this.pub_date|strftime('%Y-%m-%d') }}
-```
-
-Python datetime objects implement the standard comparison protocol, so
-ordering still works without modifying `blog.ini`:
-
-```ini
-[children]
-model = blog-post
-order_by = -pub_date, title
+        # Derives type name "asciidoc" from class name.
+        self.env.add_type(AsciiDocType)
 ```