Created a new function, session_t::reread_journal_files, which throws
away all previous state data and reads in the same files again. This is
needed to allow Emacs to communicate with Ledger via the REPL, so that
it tell Ledger when it has made changes to the user's data file.
For example, "ledger eval options.limit" prints 0 (for false), but:
"ledger -l hello eval options.limit" print "hello"s, since the value of
options.limit, once set to a value, is that string. For flag options,
such as -Y, eval prints 0 if unset, and 1 if set.
This feature allows value expressions to be conditionalized based on the
presence of user options.
