Added functionality to sort attrs for variables

[joshuatorrance]

Allow the attributes of variables to be sorted.

• New command line option, support full variable names or regex
• Allow sorting of variables
• Abstracted common functionality between global/variable attr sorting
• Tweaked verbose printing in set_attribute for variable attrs (now does varname:attrname) and to print something when trying to remove an attr that isn't there
• Fix existing tests
• Add tests for variable sorting
• Fix and tests for _FillValue not being allowed to be set as variable attr
• Update docs

Closes #99

[aidanheerdegen]

I have some questions/suggestions.

[aidanheerdegen]

LGTM