Added support for importing entries for 2.0rc1.

I wrote some infrastructure necessary to create importer plugins and then used it to create a jrnl importer plugin; --import with no argument assumes jrnl as the default (which is how I think that it should be).

The list of import plugins is currently hard-coded, but one of my other pull requests solves this problem for the exports plugins and if you are happy with it then it is no trouble for me to modify the imports to use the same system.
What about just `self.entries += set(other_entries) - entries_set`?