Adding a mark-subword function and including it in expand-region.el

As you advises from #143 I made a function to mark the subword when subword-mode is non-nil and included it in the main expand-region.el expansions.

I ran the tests with cask before the change and afterwards and there was no change. (Currently there are 3 tests failing in my machine but they seem unrelated).

I tested the expansion on a C++ project with subword mode being on and it worked as expected. With subword-mode off, it simply selected the whole word as would be the expected behaviour.