(inlineImg) Rewrite of main loop

## Utilize an object FileMeta to contain everything needed for each
file.

It keeps the original path and supplies helper functions to do
everything necessary with each file.

## Use functional style to filter the file list

* `filterOnlyImage` Filter out fonts as well as CSS files
* `filterOnlyExists` Filter out files that don’t exist
* `filterOnlySmall` Filter away large files if IE8 mode activated

## Other fixes

* Verify that `fs.exists` instead of opening and using `try…catch`
* The path verification didn’t work in my project. I added a method of
detecting the path (#52) but kept both the other methods (#50, #54) so
as not to break any earlier functionality.