Adds replace command to theme-manipulate

Also adds some tests to make the argument validations easier to
understand.
Ensure that HTTP connections are closed

There was an issue where File Handles could possibly
stick around, resulting in crashes. I believe this fixes
that.

Also adds concurrency configuration to speed up theme-watch
Merge branch 'master' of github.com:csaunders/phoenix
Merge branch 'bugfix/do_not_exhaust_file_handles'