Use mkAbsolutePath in doAppend (XMonad.Prompt.AppendFile)

[geekosaur]

Description

Enhance mkAbsolutePath to support environment variables, then use it in XMonad.Prompt.AppendFile's doAppend so filespecs like ~/NOTES and $HOME/file.txt are supported.

Checklist

• I've read CONTRIBUTING.md

• I've considered how to best test these changes (property, unit,
  manually, ...) and concluded: manually tested in ghci

• I updated the CHANGES.md file

[geekosaur]

1. Should I add this to CHANGES?
2. Should I split this in two by the two commits?

[slotThe]

1. Should I add this to CHANGES?

Yes please (both changes, I would say)

2. Should I split this in two by the two commits?

I think it works as two commits, as it's really to different things we're doing here, but if you'd prefer one commit that's fine as well

[geekosaur]

I actually meant making a separate PR for each commit, as each addresses a different issue.

[slotThe]

Oh right. I don't think that's necessary; I think we tend to think in commits more than PRs anyways

[geekosaur]

That most recent push is just a rebase.