stbt tv: Allow complex source pipelines with shell escaping

Previously if you had a stbt.conf like:

    source_pipeline=mysrc property="prop with spaces"

bash would get confused and would pass `property="prop` as `$2` and separately pass `with` as `$3` and `spaces"` as `$4`.  Using `eval` fixes this issue although it does make me slightly worried that if some bash syntax inadvertently ended up in source_pipeline stbt tv could behave oddly.

This is required for smart-tv support as, due to the complexities of image processing `source_pipeline`s can require many complex parameters.

This commit was originally part of #100 (Smart TV support) but has been broken out for easier review and merging.