Added !default to core colors

[gregtillbrook]

Small PR that adds !default to the core blueprint colors. I know this was discussed already internally by you folks but following discussion in #123 I thought Id create this PR incase you agreed with the line of reasoning there. If not, then please close/delete this PR :)

Sass lint is passing.

[palantirtech]

Thanks for your interest in palantir/blueprint, @gregtillbrook! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.

[llorca]

Thanks @gregtillbrook! Let's keep this open, other team members will review in a couple weeks after the vacation 👍

[patresk]

+1! This would be great.

[giladgray]

some relevant conversation begins here: #396 (comment)

i'm inclined to just ship this because it'll make consumption much easier for everyone. modifying the named colors will be frowned upon but i recognize that it's much simpler to just modify $blue3 instead of the many aliases that map to it.

[adidahiya]

@gregtillbrook Circle is not reporting build status -- would you mind pushing an empty commit to try and trigger CI?

[giladgray]

@gregtillbrook if we don't hear from you by end of today, I'll take over this feature so we can ship it in tomorrow's release 👍

[Santas]

Is it possible to add !default to $button-intents as well https://github.com/gregtillbrook/blueprint/blob/12835c96927a87289a76a12d1763dd2b81569a57/packages/core/src/components/button/_common.scss#L54 or should that go to a separate PR?

[isTravis]

@Santas, that also came up in the discussion on #396 and seems to have just been a missed line. @giladgray fixed it already in this commit, so I'd assume button-intents will have !default in the next release.

[gregtillbrook]

Sorry I didn't reply - was on a long haul flight. Thanks for accepting this one! :)