Add a config file

[MattWindsor91]

There are quite a lot of tunable parameters in tools like phph and litmus (especially the latter); for instance:

• whether to stop when we see a particular outcome;
• how often to rotate threads;
• how many times to run the tests (or whether we keep going until timeout);
• which barrier implementation to use;
• whether to disable checks;
• eventually, whether to preload (#10);
• eventually, how big to make the env pool (#16);
• eventually, the output format;
• eventually, the verbosity.

Because these will usually be the same across a large amount of uses of phenolphthalein, it makes sense for them to be stored in a config file with command line parameters becoming a temporary override. This issue tracks implementing that.

[MattWindsor91]

I might try using serde for this (and open an issue for serde-ing observations).