Search in all specified PATHs

This pull request makes Searcher scan all the PATHs that are given in the command line, not just the first one as before. Fixes issue #68.
Thanks for the PR.
I prefer `goroutine` to `for range` because it's faster.
And this PR breaks tests, so I have implemented same behivor on https://github.com/monochromegane/the_platinum_searcher/commit/936a8dab38badbf035dbfcb537d330417a8efa1c

Please try it :smile: