Add a scanners directory

[LivInTheLookingGlass]

The idea of a scan is that it should discover new markets to create. It should be implemented as an iterable, ideally a generator.

An example would look like (warning, pseudocode):

def scan_github(db, owner, repo, do_all = False**options) -> Iterable[CreationRequest]:
    db.execute("CREATE TABLE IF NOT EXISTS github_scans (owner TEXT, repo TEXT, number INT);")
    for pr in iter_prs(owner, repo):
        cur = db.execute('SELECT * FROM github_scans WHERE owner=?, repo=?, number=?;', (owner, repo, pr.number))
        if not cur.rowcount():
            yield create_request(pr)
            db.execute("INSERT INTO github_scans VALUES (?, ?, ?);", (owner, repo, pr.number))
            db.commit()
        elif not do_all:
            return  # I've already been here

[LivInTheLookingGlass]

Should be able to enable all, disable all (default), or enable individually