Look for hledger on start of execution

[apauley]

Run hledger --version on start of execution to determine if hledger is in the path.

Exit with a helpful error message if hledger cannot be found.

The error message we currently see when running hledger-flow on a system that doesn't have hledger installed isn't very helpful:

hledger: createProcess: runInteractiveProcess: exec: does not exist (No such file or directory)

Keep the hledger version number around so that it can be used by other parts of the program.
It will be helpful for example to show this version number when printing out the program options (issue #11).

We probably require a specific version (or later) of hledger, although I don't know what the earliest version of hledger is that will work with hledger-flow.