Add rm and strip abilities to atree.

The new line syntax is:
  [SRC] [rm|strip] DEST

This allows one to write things like this in atree:

  bin/src
  bin/src bin/dest
  bin/src "bin/another file name"

  rm dest/file
  rm dest/dir  # recursive

  strip bin/src
  bin/src strip bin/dest

Src and dest can contain spaces if full enclosed in double-quotes.
The strip command can be overridden using the STRIP env var.

Change-Id: I22aae7a87c36c082e1aab87132099a3c644914da