Add support for CMake bracket comment

[gbellizio]

It would be nice to support CMake bracket comment.
A bracket comment is a new style for comments introduced in CMake 3.0 that can be used for multiline comments. It's inspired by Lua opening long bracket (already supported by Pygments).

# inline CMake comment

#[[ multiline
CMake comment
]]

#[==[ another multiline
CMake comment
]==]

See documentation for comments and bracket argument for details.