Fix message ("Unused variable" -> "Unused function" for function identifier)

[tyru]

Fixed message for function identifer.

Sample script

function! s:foo()
endfunction

Before

vint test.vim 
test.vim:1:11: Unused variable: s:foo (see :help E738)

After

vint test.vim 
test.vim:1:11: Unused function: s:foo (see :help E738)

[coveralls]

Coverage increased (+0.03%) to 95.48% when pulling ba13d34 on tyru:unused-function into e44dedf on Kuniwak:master.

[tyru]

@Kuniwak @blueyed I forgot about true_value if condition else false_value because I don't write python so much.
Should I rewrite with it?

[blueyed]

Suggested change

	self.description = 'Unused {node_type}: {var_name}'.format(node_type = node_type, var_name=identifier_value)
	self.description = 'Unused {node_type}: {var_name}'.format(node_type=node_type, var_name=identifier_value)

[blueyed]

@Kuniwak @blueyed I forgot about true_value if condition else false_value because I don't write python so much.
Should I rewrite with it?

I think it's fine - and actually makes it clearer for code coverage.

[blueyed]

Maybe this should be split into a separate ProhibitUnusedFunction instead?