htrun: Request to add a ConnectorPrimitive base on files/stdio

Currently, htrun supports communicating with a device-under-test (i.e. one that runs greentea-client) in one of the following ways:
* serial port
* remote (e.g. RaaS)
* Fast Models (via a socket)

But if we want to the whole test setup (device & host) to be on a PC only, which is useful for integration testing for example, support for stdio/files by htrun is valuable in allowing the "device-side" program and htrun to communicate as two processes.

To implement this, add a class of [ConnectorPrimitive](https://github.com/ARMmbed/greentea/tree/master/src/htrun/host_tests_conn_proxy) and command line options for passing input/output files.

Start by inspecting src/htrun/host_tests_conn_proxy and the existing serial, remote, and Fast Models connector primitives. Then trace htrun's command-line option handling to determine where input and output file arguments belong. Done means htrun can communicate with a device-side process through stdio/files and accepts options for those files.

Written by the indexing model from the issue text.