Enhances completion for beancount-mode in the following ways:
1. Adds completion for:
- Event directives and values,
- The payee field in transactions,
- Currencies and commodities,
2. Fixes completion for #tag and ^links not working at the end of a
transaction's heading.
3. Completion now scans not only the current file, but any included
files (recursively) for candidates if `+beancount-files' is set to
`auto`. It can otherwise be set to a list of static files or set to
`nil` (reverts to only scanning the current buffer). This applies not
only to completion-at-point functions, but also interactive commands
like `beancount-insert-account'.
This is all very rudimentary and needs polish!
d3e8ca8d9d