Add full hydra support

[camall3n]

Hydra combines args and values with = (db=mysql) and optionally uses + and ~ prefixes. It also uses . and / characters for specifying hierarchical overrides.

[camall3n]

Partly addressed by +arg-mode hydra in 7acaa94, but this only really captures the = syntax, and it's a bit of a hack

[camall3n]

Related to #33