feat: allow to customise import path

[decentral1se]

Is this a fix, improvement or something else?

Improvement. Related to #124.

  -import-path string
        import path: foo.org/myproject/i18n (default "github.com/leonelquinteros/gotext")

What does this change implement/fix?

A short-cut towards allowing users of xgotext to customise the location of where their i10n function gets called from.

I was able to understand how to do this instead of an actualy full-blown --keyword argument, pending #124 (comment).

I don't think this is a good solution but you can now override the import path and then re-define a Get / GetC / etc. in your own project.

Maybe some people prefer this approach, I'm not sure.

I have ...

• answered the 2 questions above,
• discussed this change in an issue,
• included tests to cover this changes.