better ledger compatibility

[simonmichael]

Ledger and hledger are both evolving and our compatibility is not all it could be.

• I'd like to improve compatibility especially in the area of file parsing, such that a Ledger user can expect to be able run hledger on any of their ledger journal files and have it at least not fail to run, even if it ignores the unsupported parts.
• The CLI is another area to look at.
• Also, we need more systematic ongoing testing and documentation of compatibility to minimise frustration when people move between hledger and Ledger.
• Documenting commonalities with beancount and others would be a bonus.