Add ParseArgs command line flag

Adding a ParseArgs command line flag to gearcmd to support sending json
as payloads.

For example, before this change the json payload {"key":"value"} would
get parsed by bash into {key:value} (ie without the quotes) which is
invalid json. We decided the easiest way to address this was to add a
flag that allowed the user to bypass any parsing and send the raw
payload to the worker.
Why are double line breaks expected? Also, you could use backticks for more readability: ```assert.Equal(t, `{"key":"value"}\n\n`, response)```