Allow extending bfg9000 with custom rules/languages/etc

[jimporter]

It would be nice if people could add custom rules, languages, etc to bfg9000 so that they don't have to rely on me to support their use cases (especially important for really unusual use cases that I wouldn't want to add into bfg9000 proper).

The code for the builtin functions is already largely structured like this, with one file for each group of builtins, so it would just be a matter of letting users load their own modules at the same time. One way of handling this would be to look in my_project/bfg9000 and load any .py files in there before executing the build.bfg file.

[jimporter]

This is partially handled via 9ab7408fb96548842c2b613a3644e2b5edd71f87. Leaving open for the other parts.