Switch to commonmark 0.5.0

Reasons to switch:

  * Supports the CommonMark[1] "standard"
  * Fewer dependencies
  * BSD 2-clause license
  * Fast (claims 10-20 times faster than pegdown)
  * Cleaner extension API than Pegdown
  * No timeout limit required

Testing with Documentation/markdown.md:

        | cold vm | hot vm |
        +---------+--------|
  prior |   2.29s | 279ms  |
   this |   1.64s | 139ms  |

Testing with Chromium's botmap.md:

        | cold vm | hot vm |
        +---------+--------|
  prior |   3.47s | 946ms  |
   this |   1.83s | 257ms  |

Changes in Markdown from Pegdown:

  * No <<double angle>> quotes (confused with HTML).
  * No table captions (no [caption after table]).
  * Table columns are same width (no multi-column spans).
  * Smart quotes can no longer be disabled by \'.
  * Horizontal rule should be at least 3 -, e.g. ---.
    (However GitHub Flavor -- is still accepted.)

[1] http://commonmark.org

Change-Id: Ief189d0226901a284d9b86d9798310786ae1e983

README.md

diff --git a/README.md b/README.md
index 21bb473..686f964 100644
--- a/README.md
+++ b/README.md
@@ -63,20 +63,22 @@
 Code Style
 ----------
 
-Java code in Gitiles follows the [Google Java Style Guide]
-(https://google.github.io/styleguide/javaguide.html) with a 100-column limit.
+Java code in Gitiles follows the [Google Java Style Guide][java-style]
+with a 100-column limit.
 
-Code should be automatically formatted using [google-java-format]
-(https://github.com/google/google-java-format) prior to sending a code review.
-There is currently no Eclipse formatter, but the tool can be run from the
-command line:
+Code should be automatically formatted using [google-java-format][fmt]
+prior to sending a code review.  There is currently no Eclipse
+formatter, but the tool can be run from the command line:
 
 ```
 java -jar /path/to/google-java-format-1.0-all-deps.jar -i path/to/java/File.java
 ```
 
-CSS in Gitiles follows the [SUIT CSS naming conventions]
-(https://github.com/suitcss/suit/blob/master/doc/naming-conventions.md).
+CSS in Gitiles follows the [SUIT CSS naming conventions][suit].
+
+[java-style]: https://google.github.io/styleguide/javaguide.html
+[fmt]: https://github.com/google/google-java-format
+[suit]: https://github.com/suitcss/suit/blob/master/doc/naming-conventions.md
 
 Code Review
 -----------