Changed call to EDITOR to allow for parameters

The EDITOR environment variable is used to create and edit files in the vault.
But if the EDITOR variable contains parameters, subprocess.call() breaks.

This fixes the EDITOR environment variable to be safely split into a list.
It adds a dependency on shlex.
Merge pull request #6580 from ramondelafuente/fix-vault-editor-call

Changed call to EDITOR to allow for parameters