Made customization easier to understand

Split semanticColors into gui and term variants. Slightly less efficient, but also much less confusing.
Only setting the hi color for the appropriate type of vim (gui vs term)
No longer overwrites attempts to set semanticColors in vimrc
Updated README


These are pretty much just small changes for easier configuration (It took me too long figuring out how the colors were set for terminal vim). I feel like there isn't a need for there to be the same number of gui and term colors, because for someone like me who only uses terminal vim, I just want to make a list of colors that I am going to use and not care about the list that I won't. Also made the plugin not override any values that are set in the user's vimrc, which makes automatically setting up the plugin easier.
Hope you approve of the changes and accept the pull request.