Fix to restrict colon after name in text directive

Related to remarkjs/remark#651.
Closes GH-9.

Reviewed-by: Christian Murphy <christian.murphy.42@gmail.com>

lib/tokenize-directive-text.js

@@ -40,7 +40,9 @@ function tokenizeDirectiveText(effects, ok, nok) {
   }
 
   function afterName(code) {
-    return code === 91 /* `[` */
+    return code === 58 /* `:` */
+      ? nok(code)
+      : code === 91 /* `[` */
       ? effects.attempt(label, afterLabel, afterLabel)(code)
       : afterLabel(code)
   }

readme.md

@@ -148,11 +148,11 @@ He dies.
 The `name` part is required.  The first character must be a letter, other
 characters can be alphanumerical and `-`.
 
-The `[label]` part is optional (`:x` and `:x[]` are equivalent).
+The `[label]` part is optional (`:x` and `:x[]` are equivalent)†.
 When used, it can include text constructs such as emphasis and so on: `x[a *b*
 c]`.
 
-The `{attributes}` part is optional (`:x` and `:x{}` are equivalent).
+The `{attributes}` part is optional (`:x` and `:x{}` are equivalent)†.
 When used, it is handled like HTML attributes, such as that `{a}`, `{a=""}`,
 , `{a=''}` but also `{a=b}`, `{a="b"}`, and `{a='b'}` are equivalent.
 Shortcuts are available for `id=` (`{#readme}` for `{id=readme}`) and
@@ -161,6 +161,11 @@ When multiple ids are found, the last is used; when multiple classes are found,
 they are combined: `{.red class=green .blue}` is equivalent to
 `{.red .green .blue}` and `{class="red green blue"}`.
 
+† there is one case where a name must be followed by an empty label or empty
+attributes: a *text* directive that only has a name, cannot be followed by a
+colon. So, `:red:` doesn’t work. Use either `:red[]` or `:red{}` instead.
+The reason for this is to allow GitHub emoji (gemoji) and directives to coexist.
+
 Containers can be nested by using more colons outside:
 
 ::::spoiler

test.js

@@ -60,6 +60,12 @@ test('micromark-extension-directive (syntax)', function (t) {
       'should support a dash in a name'
     )
 
+    t.equal(
+      micromark(':a:', options()),
+      '<p>:a:</p>',
+      'should *not* support a colon right after a name'
+    )
+
     t.equal(
       micromark(':a[', options()),
       '<p>[</p>',