Add support for case-insensitive attribute selectors

[murrayju]

According to Mozilla:

[attr operator value i]
Adding an i (or I) before the closing bracket causes the value to be compared case-insensitively (for characters within the ASCII range).

This appears not to be supported by Less currently, and results in a compiler error.

expected ']' got 'i'

[seven-phases-max]

Oh, no: changing labels since it's not in CSS3 (just a CSS4 draft).

[murrayju]

Yep, it is in working draft, but browsers seem to support it already (tested in Chrome 55 and Firefox 51).

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.