Add UnnecessaryInterpolation linter

Add linter which checks for unnecessary uses of interpolation in inline
content for a tag.

For example, the following two snippets are equivalent, but this linter
will recommend the first be converted to the second since it is more
concise:

    %tag #{expression}
    %tag= expression

Change-Id: Ia8aa271b3c20e70317ef162e3023213fb97844f9
Reviewed-on: http://gerrit.causes.com/42292
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>