Allow user to set custom colors

[mtorres10]

Current list of colors is limited, but there is the possibility to add a custom color.
Adding custom colors will allow users to get closer to UWP apps, as we could get the OS colors and make the metrocontrols use such color.

The idea behind this request to be able automatically grab OS accent colors and use them in metro framework.

@dennismagno This is why I created this pull request #129, please let me know your thoughts:

This is an example, changing OS colors which also reflects the change in metroframework (PR does NOT include this code, as of now, it is only a proof of concept.)