[Linter] The public_header_files attribute should allow only headers

[fabiopelosin]

Originally reported in #44 (comment)

[fabiopelosin]

The list of the header extensions can be copied from here and the logic should be added in a new method stored here: https://github.com/CocoaPods/Core/blob/master/lib/cocoapods-core/specification/linter/analyzer.rb#L17.

This validation should be performed in the consumer.header_files and the public_header_files checking that the extension is included at the end of each pattern.

[fabiopelosin]

This should be moved to CocoaPods as the files need to be fetched to perform robust valuation as patterns like the following could be hard to handle:

classes/**/*.{h,hpp}