Custom color schemes

I externalized the configuration for the themes to outside the `plugins` so it doesn't get automatically executed.

Now, on `yadr-appeareance.vim` you just set your desired colorscheme name in the variable and then it will also load the customization file based on that variable.

The second commit to this PR is my personal colorscheme `marelo`. So if wanted to change from solarized to my personal color scheme, all I had to do was to put "marelo" inside the variable. Naturally, feel free to leave the last commit out if you want.

Thanks!