More context-aware ledger pcomplete rules.

Provide completions based on information retrieved with 'ledger-context-at-point'.
To support step-by-step completion of transactions some more patterns were added
to 'ledger-line-config'. The actual completion rules mostly reuse some of the
convenient input commands that are buried in ledger-mode. Using the standard
pcomplete termination string (" ") to allow a "context switch" at the end of each
completion step.