Some people are using GitJournal to edit Hugo websites, this option is
for them as the Hugo front matter uses the 'lastmod' key to indicate
when the post was last modified.
Possible Options - modified, mod, lastModified, lastMod, lastmodified or
lastmod. When read the the note we will figure out which one is being
used and accordingly write back that value.
This makes it easier to use GitJournal for editing Hugo websites as they
usually use the 'lastmod' field to indicate when the post was last modified.
This is so that we can start configuring what the yaml metadata keys
should be called, and not have them hardcoded.
This also allows the note.created to be null. Earlier we were always
setting it to some value. That's doesn't seem right.
