Added ability to free

[JoNil]

Adds a free function so you can use nfd from a dll on windows that uses a different version of msvcrt and thus a diffrent heap than your main application.

Cheers and awesome library! :)

[mlabbe]

I'm aware of the static library limitation of NFD; thanks for taking a step in the right direction. Did you manage to update the Premake scripts to optionally generate DLLs? It seems that this PR makes sense in the context of a build feature that doesn't yet exist.

I'd happily take a build script PR that adds DLL support.

[mlabbe]

Hey, I'm wondering if you want to weigh in here on my question. I'm going to release the next version of NFD in a couple weeks; we've had some updates I'll be moving from the devel branch to master. If not, no worries.

[mlabbe]

PR is good but not comprehensive; a larger update to address the shared library is planned.