Reading entire plugins

[benjaminwinger]

I know that you say in the README that esplugin focuses on providing an API to libloadorder and LOOT, rather than being a general-purpose plugin parser, but would you be interested in contributions that would make it more general purpose?

One thing worth noting is that I'm doing this from the perspective of Morrowind/OpenMW, and while it would be great to implement general purpose reading and writing of plugins for all formats supported by esplugin, I also quickly realized that writing in support for the other formats would require significantly more work, and I'm not sure that I'm the best person to be writing the code for the other formats, as I likely won't be actually using it.

It's also worth noting that I plan to eventually add support for modifying and writing plugins.

Basically there were two options I was considering:

1. Implement and merge general purpose code for Morrowind/OpenMW plugins only, with the possibility of extending it to the other formats in future.
2. Create a derivative library that focuses on being a general purpose Morrowind/OpenMW plugin library.

I prefer the first option, as keeping code together would put more eyes on it, but if you prefer I can move this work to a derivative project.

Note that the code included in the merge request was fairly hastily thrown together and will need some changes before it can be merged (e.g. read_plugin is currently broken for games other than Morrowind, and I haven't written any tests for this). This pull request is mostly just to get your input, but I thought I should include the changes I've made so far.