Add validation based on MapServer version

[geographika]

Allow validation against different versions of MapServer, for example validating Mapfiles for MapServer 7.0, or for 7.2.
A new version parameter can be passed to the Validator class or used on the command line.

[coveralls]

Pull Request Test Coverage Report for Build 475

• 35 of 43 (81.4%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.004%) to 91.529%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
mappyfile/utils.py	1	2	50.0%
mappyfile/cli.py	1	8	12.5%

Totals
Change from base Build 468:	0.004%
Covered Lines:	1167
Relevant Lines:	1275

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💛 - Coveralls