Add tool for easily getting an old version of filer

[humphd]

For our migration testing and tools/fs-image.js, it's necessary to be able to pull old versions of our filer bundle out of git.

I'd like a tool that works like this:

$ tools/get-filer-version v0.0.44
Wrote filer-v0.0.44.js

$ tools/get-filer-version d66114e
Wrote filer-d66114e.js

This could then be used with fs-image to create a filesystem image using an older version of filer:

$ tools/get-filer-version v0.0.44
Wrote filer-v0.0.44.js

$ fs-image files/ --filer .filer-v0.44.js

The tool can basically take the argument you pass it, assume it's a SHA-like value (SHA, branch, tag) and then checkout dist/filer.js at that commit.

[humphd]

Fixed. See https://github.com/filerjs/filer/blob/master/tools/get-filer-version.js for details on how this works.