add precision up to 6 pairs in maidenhead

[cedricdelayre]

This modification aims to add more precision to maidenhead's code. Up to 6 pairs like JN19CF83FC56 is now possible!

References:

• Wikipedia
• Perl Ham Locator's code
• Earth Point | Tools for Google Earth

[mykhal]

I've discovered this lib today and I'm glad someone is fixing the issue, as IARU has "allowed" extension to (at least?) 10-character locator string years ago.

(Didn't check the code, but obviously tests have to be touched.)