Fetch: support custom http headers

[mhuang74]

Support setting one or more custom http headers for fetch (#77):

• primary usecase is to set api keys. since each api requires different keys, it seems simpler to just set via args and not bother with config files for now
• to support config file, qsv should probably just have one single config file with command specific sections, and maybe even support profiles
• header format "key:value" uses ':' colon as separator
• integration test uses http://httpbin.org/get to confirm that custom headers are passed to server (thanks @jqnatividad for suggesting this)
• fixed bug with white space in URL column