Adding context variable to custom function calls.
Renamed `rem-fallback` plugin to `rem` and expanded it to work in different
modes and optionally affect all properties not just font related.
Renamed `px2rem` plugin to `px2em`.
Added a context argument to the compile method.

The context is either `file` or `filter`, gives plugins a way to negotiate situations where a file would be written but is not desired; e.g. when using Crush as a filter.