Consistently quote arguments when running hledger

[shivjm]

There are many places that call hledger, and for the most part, the arguments are a simple string. This usually isn’t an issue, but, for instance, running M-x hledger-run-command RET balance RET some account with spaces produces something along the lines of hledger balance some account with spaces, i.e. four separate arguments to the balance command. It would be good to consistently escape user input (unless it’s specifying hledger options, of course).

Sort of related to #50.