Remove extra quoting and instead add a backslach to each character in
the extra args before appending it to the command.
ex. atlantis plan -- -var key=val will result in:
sh -c "atlantis plan \-\v\a\r \k\e\y\=\v\a\l"
Fixes#697.
This version changes how we parse custom run steps.
Previously we used the go-shlex library to parse the steps but this
caused users issues because they'd have to use weird escaping.
Now we simply pass the step directy to sh -c without any parsing. The
parsing was unnecessary.
