[generate-flixel-swc.sh] Library path optional

Previously, an error message would be thrown if the `lib` folder did not exist.
Now it is entirely optional.
Rename shell scripts for general use

The scripts are (almost) no longer "flixel" specific and can be re-used elsewhere.
Just a few final changes need to be made to the actual scripts.
[generate-swc.sh] Add error message on failure

Will display an error message if unable to retrieve the version
number from `git describe`.

Also allows the user to manually define a version number,
rather than have one automatically generated for them.
[generate-swc.sh] Use array instead of string for arguments

I was actually getting an error when using a library folder due to a
misunderstanding of how shell handles strings.

See the following page for an explanation:
http://mywiki.wooledge.org/BashFAQ/050#I.27m_constructing_a_command_based_on_information_that_is_only_known_at_run_time
Merge pull request #135 from IQAndreas/ref-utils

Refactor `utils` for general use