Add experimental new OFX importer (#921)

Hi there, 

I try to tackle #798 here. It was suggested to throw this behind a
feature flag, so here it is!

this does its best to import the problem file in #767. 

I am working on this because it would make my work on #918 easier :)

Feel free to set the feature flag to true and try the new importer. The
date parser is not as sophisticated as the one in `node-libofx`, but I
tried 3 different OFX files, one from my bank, one from the mocks, and
one from #767. They all seem to work well enough on that front, but this
is definitely the weak point of the new implementation.

Let me know what you think!