allow options such as -Xloggc:file/path and -Xdock:name="thing" to be used

Previous implementation stripped the -Xloggc:value down to -Xloggc which
is an invalid option.
Improving the handling of -Xthing:value options to prevent duplicates

Separating the option on a possible : and setting the key to Xthing:
and value to the remaining allows for the args.rb to maintain the
hash of these options.

This has the added benefit of always setting @value to be something
even if that is an empty string, thus the to_s method can be simplified
greatly.
Merge pull request #9 from gid79/nonstandard_options_bug

allow options such as -Xloggc:file/path and -Xdock:name="thing" to be used