New: space-in-parens rule

Closes #627.

You'll notice that I set the rule to off by default. This is to be consistent with the existing `space-in-brackets` rule which seems closely related as more of a stylistic concern. I can easily switch it back to on by default as @nzakas originally said; however that will also require either setting it to off in eslint's .eslintrc or correcting the 20 or so files that currently fail it.