Issue #10: Accept -q/--quiet option - don't use it yet
Issue #10: Implement --quiet, v1: avoid most output

Messages like "process file X.pts" are still output. See next commit.
Fix #10: Make pts completely quiet when requested

This requires moving the "process file X" into another function (which
has access to the options through a reader monad) and writing a
reusable outputLine function - a variant of putStrLn which is disabled
by --quiet, and runs in this reader monad.
Add missing type signature
Merge pull request #12 from Blaisorblade/issue/10

Fix #10: Make pts completely quiet when requested by new --quiet option