Configurable extra runsBefore and runsAfter phases

[pbatko]

Currently scalac-scoverage-plugin runs after typer and before patmat phases. There are no other phases in between so the order is unique and fixed.

Situation changes when using other scalac plugins which add phases in between typer and patmat. Scoverage phase can be then nondeterministically inserted somewhere between these new phases.

To fine-tune phases ordering I propose adding following flags

-P:scoverage:extraAfterPhase:<phaseName>
-P:scoverage:extraBeforePhase:<phaseName>

which would specify what other phases beside typer and patmat need to taken into account when determining phase ordering by appending them to runsAfter and runsBefore lists.

PR: #164

What's your comments? Can we have these flags added?

[sksamuel]

This is now merged.

[gslowikowski]

What about documentation?