Fix Issue 673: Parse error with named CSS grid lines and areas

[blizzardengle]

This patch protects CSS named grids by replacing the brackets before any parsing or compiling happens. Once all parsing and compiling is done the brackets are replaced.

NOTE Brackets are changed to 3 underscores [grid-name] => ___grid-name___ which works great but has a small chance of clashing with someones code, in this case I would suggest changing it to 4 underscores. Who is writing that many in their less/ css?