Fix typo in Windows host setup instructions
Add missing formatting to command line example
Add section about GPU blacklisting

Fixes https://crosswalk-project.org/jira/browse/XWALK-1822
Add a note that "cordova plugin" should be used for migrated projects

One user had some issues when they tried to use plugman directly
on a Cordova project they had migrated to Crosswalk.

Add a note that the recommended method for adding plugins to a
migrated project is via "cordova plugin add".

Relates to https://crosswalk-project.org/jira/browse/XWALK-1842
Make replace_version_string global

This is so I can use it in the manifest page to get the current
major version number.
Add manifest overview page