Support -files flag to specify files explicitly

Instead of input files or dirs specified after the options, read
paths of the files to process from this file, one per line.
If '-' is specified, stdin is read.

The short options is -L to make it compatible with Exuberant Ctags.
Bug fix: now --no-vars option works correctly.

'variables' must come first in the alternation to match the
key in %options.
Add tests for --no-vars option and its variants.
Added tests for the new -L/-files option
Merge pull request #3 from dtikhonov/master

Fixed -no-vars and added -files option