Support multiple positionals

[aherrmann]

Add support for multiple positional parameters - display such parameters correctly in the help and usage messages.
Adds unit tests for the help and usage message.

[Hejsil]

I'll merge this, as it is an improvement for help and usage. Do note though, that parse will not handle this correctly in certain cases. For example <u32> <string>... will make the positionals field in the result be []const u32, ignoring that the other positional parameter specified string.

[aherrmann]

@Hejsil thank you! And thanks for pointing out the current limitations. My current use-case only involves string parameters, so I didn't notice this limitation. I've opened #98 to keep track of this limitation.