Consider optional installs for file formats

[Libbum]

At the moment we have yaml, json, ron, messagepack and csv formats - each require a library to pull in and compile. None of them really need too much overhead, but consider that users will probably only ever use one or two formats.

If we make some optional, or make a build script that enables/disables options, then users can skip some of the compiling bloat.

[Libbum]

This can be done with features and shouldn't actually be too hard to implement. May even be fine for the v0.1 milestone. I just don't want the install process to be difficult.

[Libbum]

The issue with doing this is that people may not be able to run config files that they don't have interpreters installed for. Plus we would have to dump if users select options they haven't installed, then they'd have to recompile if they have an input file they can't run etc.

It may be best to keep this at v0.2 to see what people want.