Add SLP support

[OverlordQ]

Should verify that the address conversion is actually right.

[coveralls]

Pull Request Test Coverage Report for Build 15

• 27 of 29 (93.1%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.4%) to 92.694%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
cashaddress/convert.py	27	29	93.1%

Totals
Change from base Build 14:	-0.4%
Covered Lines:	203
Relevant Lines:	219

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💛 - Coveralls

[ghost]

Thanks for adding tests with this. With adding SLP, my thought is that it'd be nice to have a separate functions here. Maybe like, validate_slp_address or validate_pkh_address, or something like that.

Otherwise might get a simpletoken address and try to process it while not being able to support it, even though the address itself is technically valid.