I18n support

[tstrachota]

• UsageError split into more specific exceptions with parameters. It will enable projects using Clamp to catch such exceptions and print their own translated strings.
• Heading in HelpBuilder extracted to a function to make overriding easier.

[mdub]

Thanks Tomas.

I'm just wondering: is there good reason to introduce the finer-grained exceptions for reasons other than i18n? If not, perhaps it would be simpler if Clamp supported a translation-table for exception messages. You could then localise by overriding them:

Clamp.messages[:too_many_arguments] = "of margir rök"

Would that be easier than having a wrapper that caught exceptions to generate localised messages?

[tstrachota]

My primary motivation was translations. I guess a table/Hash would work as well. I can re-write the patch if you prefer the table style.

[mdub]

Yes, let's try the translation table approach, and see how it looks. If you're happy to code it up, that would be great.

[tstrachota]

Ok. I'm closing this one and opening #43