Added check to ignore --date-format option for hledger users

[GLakovnik]

This PR intends to improve ledger-mode compatibility for hledger users.
It requires no additional configuration and is completely invisible to ledger users while fixing the most common error hledger users see.

It's "the dumbest thing that works", a simple check whether ledger-binary-path points to hledger. If it does, ledger-exec-ledger does not include the --date-format option (which hledger doesn't have).

Solves the largest problem for hledger users and is a prerequisite for other improvements (WIP, i plan to make reconcilliation work as well and this is the first step.)
I've documented the incompatibilities in hledger's documentation.

With this change the following now works with hledger:
Quick balance display (<C-c> <C-p>)
Add transaction (<C-c> <C-a>)
Display ledger stats (<C-c> <C-l>)

[bcc32]

Does hledger really not support setting a format for dates? I'd rather do something different than just "silently don't respect the customized value of --date-format" in that case---perhaps we could support setting ledger-default-date-format to nil, which hledger users would have to do?