Allow overrides to color schemes

The ability to override color scheme settings ala CSS would be very convenient. I'd like a mechanism for "sub-schemes" that override only portions of the main, "active" color scheme.

For example, very few color schemes support Markdown and I don't like the looks most of those schemes in other syntaxes, like Python. I can fork my own version of schemes I otherwise like and patch in Markdown support, but then I'd have to maintain that fork. I could use the sub-scheme mechanism to create Markdown-specific overrides and add them to my syntax-specific settings like:

    {
        "color_sub_schemes": ["Packages/My Subthemes/Blue Markdown links.subscheme"]
    }

Those sub-schemes would "patch" whatever color scheme I'm currently using - say Monokai - with just a couple of setting overrides.

This could be a big help to people maintaining language support as they could include a couple of subschemes with their packages like "Mylanguage (bright).subscheme" and "Mylanguage (dark).subscheme" so that users could be quickly up-and-running with their usual, familiar color scheme.

This could also be an accessibility win for users who need a particular setting in place across all color schemes, but otherwise enjoy the wide variety of available ST2 schemes. They could add something like:

    "color_sub_schemes": [
        "Packages/Headings/Bold Headings.subscheme",
        "Packages/Headings/Inverse Italics.subscheme"
    ]

to their global settings so that they can use the same color schemes we all know and love but with convenient accomodations of their personal needs.