Implement parameterize-able color palette

[celestehorgan]

Users should be able to do the following:

1. Add a sass variable, i.e.

$main-color: #FFFFFF

Alternately, this might happen in config.toml, or a .yaml file:

[[colors]]
main: #FFFFFF
secondary: #000000

After which, we should use Hugo template variables in custom.sass to do the following:

1. Run though a loop for each defined variable
2. Define three classes: main-text (sets CSS color: ), main-bg (sets CSS background-color:), main-border (sets CSS border-color).

Allowing people to use custom colors as utility classes in their layouts.