Add support to powerline themes to override foreground color

Description

This change allows the override of foreground color for the powerline themes. You can change the foreground color by settings POWERLINE_PROMPT_FOREGROUND_COLOR. It still retains the default of using the terminal foreground color.

Motivation and Context

For people that use themes like solarized dark, the contrast between the standard terminal text and certain backgrounds makes it very hard to read. This allows for changing the foreground text to something like '15' (bright white) making the prompt much more legible.

How Has This Been Tested?

This has only been tested locally. You can see in the screenshot the default behavior and after changing the foreground color to '15' in my .bashrc.

Screenshots (if appropriate):

Types of changes

[ ] Bug fix (non-breaking change which fixes an issue)
[X] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

[X] My code follows the code style of this project.
[X] If my change requires a change to the documentation, I have updated the documentation accordingly.
[X] I have read the CONTRIBUTING document.
[X] If I have added a new file, I also added it to clean_files.txt and formatted it using lint_clean_files.sh.
[X] I have added tests to cover my changes, and all the new and existing tests pass.

Bash-it / bash-it