homectl: Add flags to edit blob directories

homectl: Add flags to edit blob directories

This makes it possible to edit blob directories using homectl. The
following syntax is available:

* `--blob-directory=/path/somewhere`: Replaces the entire blob directory
with the contents of /path/somewhere

* `--blob-directory=foobar=/path/somewhere`: Replaces just the file
foobar in the blob directory with the contents of /path/somewhere

* `--blob-directory=foobar=`: Deletes the file foobar from the blob
directory

* `--blob-directory=`: Resets all previous flags

* `--avatar=`, etc: Shortcuts for `--blob-directory=FILENAME=` for the
known files in the blob directory