Enable .hhconfig to control hh_server flags

Summary: hh_client now reads .hhconfig to see if it defines an external
program that determines which command-line options to pass hh_server when we
start / restart it.

The config file parser is pretty dumb; we could replace it with some more
intelligent external library at some point.

The integration test has been modified to launch hh_server via hh_client
(instead of doing it directly.) Thus it tests that the new .hhconfig feature
is actually working.

Reviewed By: @jwatzman

Differential Revision: D1663418

Signature: t1:1663418:1415661435:febce9fc252210d903a0f461e7d52b9527941753
Add test for autocomplete.

Reviewed By: @jwatzman

Differential Revision: D1667693

Signature: t1:1667693:1415402748:9026ef1ef085fd0b538ef1d19c370ebfc036277b