Add functionaility to query via the Accounts Endpoint

[kprinssu]

Thank for you writing up a quick Python library to access Simplefin's API. The accounts endpoint offers three parameters that we can use to query transactions:

start-date - unix timestamp
end-date - unix timestamp
pending - 1 or 0

This will allow for finer control over what transactions we can query and retrieve. I can write up a quick PR adding these in, if you need some help.