CAN should ignore 'beneath the fold'

CAN's scratchfiles may have a marker (---) called the fold. Anything beneath this point should be ignored by can. This gives the user space to keep short-lived working drafts while they explore their notes.

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• CAN should parse the scratchfile when it is updated
  • In order to be able to store notes and use content-addressable storage, CAN needs to have access to the note's content. Because of this, CAN should parse the scratchfile whenever it is updated (CAN should fire a SCRATCHFILE_UPDATED event when the scratchfile is updated). Once the AST has been constructed, CAN should ignore 'beneath the fold'.