Fix handling of escaped quotes in CMake lexer

[pyrmont]

As identified in #1452, Rouge's CMake lexer doesn't handle escaped quotes correctly in unquoted arguments. It incorrectly treats the escaped quote as beginning a quoted argument. This PR fixes that error.

It will close #1452.

[pyrmont]

@danilaml I'm still not sure I've understood the CMake language spec correctly. The code now tokenises \" in an unquoted argument as Text. In practice, it looks like this:

Is that what it should look like? It's still interpolating the argument inside the escaped quotes (which GitHub doesn't do).

[danilaml]

@pyrmont It seems correct to me. At the very least, it's much better than highlighting everything past \" and until some unescaped " as a string.
I think GitHub just uses the regular text style for ${} args, so they don't stand out when they are in an unquoted context, contrary to

message("This ${arg}")

[pyrmont]

@danilaml Cool. I realised the screenshot was a little misleading: I had highlighted foo but that's not something Rouge is doing. In any case, will merge this in now. Thanks for the contribution! :)