Refactor utility functions out of library

[edawson]

I've made a lot of improvements to the pliib, which holds a lot of the utility functions for strings used in mkmh. At some point, I'd like to refactor mkmh to rely on pliib, rather than carrying multiple versions of the same functions around. This will mean users will have to install pliib (which is also a single-header lib), but it should make the process of updating string manipulation functions a lot easier.