When we don't know a file's format, instead of choosing a subset of
readers based on content sniffing, now we just try them all.
Also, LedgerReader is now used only as a last resort,
as it's not yet competitive with JournalReader.
This reader is used by default for files with suffix .ledger or .l,
and tried along with the other readers for files of unknown type.
Currently only the bare minimum of the raw parsed data is used:
transaction dates/descriptions and posting accounts/amounts,
with the rest being ignored.
Amounts are parsed the same way as in the hledger journal format.
Malformed amounts might be ignored instead of error-reported.
