Support converting more TextMate theme styles

[kevinsawicki]

Support converting https://raw.github.com/itsthatguy/theme-itg-flat/master/itg.dark.tmTheme style themes where the main colors aren't the first element in the array.

• Look for main block without a name or scope to determining the "core settings" of the theme that are translated to variables
• Throw an error if the 6 required variables aren't present because if any are missing the generated theme will not be valid LESS since it will have left-over `{{..}`` markers in it.

Tested that is was still compatible with these style themes as well: https://raw.github.com/chriskempson/tomorrow-theme/master/textmate/Tomorrow-Night-Eighties.tmTheme

/cc @benogle

[nathansobo]

Looks good.

[benogle]