Change “Toggle comment” to be more strict when checking the TM_COMMENT_DISABLE_INDENT variable

If a bundle by default set `TM_COMMENT_DISABLE_INDENT` to `'yes'`, then it could not be disabled through use of a .tm_properties file or by the “Variables” section in the Preferences pane.

For example the CSS bundle sets TM_COMMENT_DISABLE_INDENT to `yes`. With this change I can now place the following in a .tm_properties file:

    [ source.css ]
    TM_COMMENT_DISABLE_INDENT = no