Increase modularity

[sbeyer]

It would be nice if users could register reporters, failure formatters, color themes (cf. #90), assertion framework adapters, etc. without interfering with the bandit code base.

edit: To be a little more elaborate: The idea is to split off reporters, formatters, colorizers from the options and have functions like register_reporter(), register_formatter(), etc. However, there must also be a register_defaults() that registers all the things bandit is shipped with.
One of the benefits would be that there is no need to include these reporter/formatter/… header files in every spec source file, but only in the main.cpp file, so this improves compile time a little (ignoring the fact that probably the longest compile time comes from snowhouse).