Provide standardized options to control common elements in themes

[khughitt]

Many themes include some common elements such as including the current ruby version when rvm is installed (e.g. bira).

Currently, there is no way to customize the appearance of such common elements, short of editing the theme files or manually overriding components used in the themes.

It could be useful to provide a few global theme settings to give users control over such shared elements, when they appear are used in themes.

For example, something along the lines of theme_include_rvm could be checked by each theme that includes ruby version and used to determine whether or not to include that component in PS1.