Ben Gamari authored Feb 28, 2017 and Ben Gamari committed Feb 28, 2017

This introduces a RTS option, -po, which allows the user to override the stem
used to form the output file names of the heap profile and cost center summary.

It's a bit unclear to me whether this is really the interface we want.
Alternatively we could just allow the user to specify the `.hp` and `.prof` file
names separately. This would arguably be a bit more straightforward and would
allow the user to name JSON output with an appropriate `.json` suffix if they so
desired. However, this would come at the cost of taking more of the option
space, which is a somewhat precious commodity.

Test Plan: Validate, try using `-po` RTS option

Reviewers: simonmar, austin, erikd

Reviewed By: simonmar

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D3182