Sequence diagram fixes & improvements

This commit fixes some bugs, and I believe, improves upon the current
implementation.

In no particular order, it adds:

1. Control over note font size, family and alignment (now defaults to
center)
2. Dynamic actor resizing - actor's width will now scale if its
description is bigger than the static configured width
3. Dynamic actor margins - the margin between actors will now be
dynamically calculated by taking into account the width of connecting
messages or notes
4. Fixed a small visual annoyance where a loop arrow would intersect
with the text it loops on
5. Fixed a bug where if global config -> fontFamily wasn't defined, it
would override the actorFontFamily with an undefined
6. Removed some stale / commented out code
7. Added missing config variables to the global config object in mermaidAPI.js
8. Added messageFontSize, messageFontFamily to control message (non-note)
font settings
9. Memoized the actor widths in a pre-calculation that takes notes and
signals lengths into account
10. Removed redundant console.log lines
11. Extracted out actor width & margin calculation to getMaxMessageWidthPerActor, and
calculateActorMargins

src/mermaidAPI.js

@@ -270,7 +270,42 @@ const config = {
      * This will show the node numbers
      * **Default value false**.
      */
-    showSequenceNumbers: false
+    showSequenceNumbers: false,
+    /**
+     * This sets the font size of the actor's description
+     * **Default value 14**.
+     */
+    actorFontSize: 14,
+    /**
+     * This sets the font family of the actor's description
+     * **Default value "Open-Sans", "sans-serif"**.
+     */
+    actorFontFamily: '"Open-Sans", "sans-serif"',
+    /**
+     * This sets the font size of actor-attached notes.
+     * **Default value 14**.
+     */
+    noteFontSize: 14,
+    /**
+     * This sets the font family of actor-attached notes.
+     * **Default value "trebuchet ms", verdana, arial**.
+     */
+    noteFontFamily: '"trebuchet ms", verdana, arial',
+    /**
+     * This sets the text alignment of actor-attached notes.
+     * **Default value center**.
+     */
+    noteAlign: 'center',
+    /**
+     * This sets the font size of actor messages.
+     * **Default value 16**.
+     */
+    messageFontSize: 16,
+    /**
+     * This sets the font family of actor messages.
+     * **Default value "trebuchet ms", verdana, arial**.
+     */
+    messageFontFamily: '"trebuchet ms", verdana, arial'
   },
 
   /**