Integrate methods of getting account lists

[alerque]

Now that #78 is merged there are two different systems around for generating lists for completion: one for the omnicomplete sources (the more configurable and robust options) and a different one for the command line report completion (hard coded with a single source).

These should be integrated somehow. The variable scope and cache update triggering will need some work for this to happen, but completion inside the buffer for the account name in postings should be the same as :Bal <account>, and the same for transaction descriptions and payee reports.