Sort through hledger-vim plugin for ideas and bring to parity

[alerque]

The hledger-vim plugin is not as extensive as this one but it does have a few interesting bits. Per this discussion the author has noted that it is GPL, and hence compatible with this plugin, and also that he might migrate and deprecate the plugin if this one was feature compatible.

It seems like a worthwhile effort to:

• [ ] Sort though the list of features for ones that are duplicate but implemented differently for any that are better.
• [ ] Sort though the list of features for ones that are not implement at all and copy or re-implement them as appropriate.
• [ ] When there is feature parity, write a migration guide to users of that plugin and submit it as a "sunset" notice PR.