Add parsing of gutterSettings

[timhillgit]

Many TextMate2 themes use gutterSettings to store colors for the gutter area. This is not supported by Sublime Text. This PR enables using gutterSettings to set the foreground and background of the gutter but only if they are not already set in settings.

Closes #469

[keith-hall]

Looks good to me. Probably it would also make sense to update the unit tests at

syntect/src/highlighting/theme_set.rs

Line 87 in d3f6d22

fn can_parse_common_themes() {

to prove it parses it as expected.

[timhillgit]

Ah, that's a good idea, I was only looking at theme_load so I missed it. I'll add that in.

[timhillgit]

@keith-hall Are there any more unit tests or anything else you'd like to see before this is merged?