Add example and parsing for mixed attribute pairs

[geographika]

OFFSET and POLAROFFSET allow the following attribute pairs [double|attribute] [double|attribute] - combinations of bound attributes and values. This pull request allows these to be parsed correctly. See #114.

[coveralls]

Pull Request Test Coverage Report for Build 496

• 4 of 4 (100.0%) changed or added relevant lines in 2 files are covered.
• 4 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.02%) to 90.994%

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Files with Coverage Reduction	New Missed Lines	%
mappyfile/transformer.py	4	92.57%

Totals
Change from base Build 494:	0.02%
Covered Lines:	1172
Relevant Lines:	1288

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 512

• 4 of 4 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 91.537%

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Totals
Change from base Build 509:	0.02%
Covered Lines:	1179
Relevant Lines:	1288

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💛 - Coveralls