ignore md files with a leading dot '.' as filename

As mentioned on issue #56, you would got a WSOD when you have a markdown file left open in your editor. It appeared it was because Phile also records files that were prepended with dot in its filename. 

For example, you have two files `index.md` and `.index.md` (VIm creates a backup with the same filename prepending with a dot and adds a `.swp` extension in most cases), however either Phile gets confused about the content for `/index` (does it load the `index.md` or `.index.md`?) or either it's because the URL of the latter one is invalid (I've never seen `/.index` URL before)

Anyway, this PR solves #56 by only recording filenames that are *not* prepended with a dot `.` character.