Add improved file loading API (feedback wanted)

[fallahn]

By default tmxlite uses fstream to open tmx files. As noted in #76 this does not work on android, the current work-around being loading the entire file into a string and passing to Map::loadFromString().

In #80 it is also noted that there is currently no support for zstd compressed maps as this requires including external dependencies.

The idea then is to create an API for tmxlite which allows a user to define a file loading mechanism of their choice. tmxlite itself could include file loading functions for android, for example, as well as optional file loaders for zstd and other potential sources such as PhysFS.

If anyone has any feedback on this idea, or ideas for API design, please comment! The current thinking would be similar to that of SFML's InputStream.