Screener pagination

[fabioDMFerreira]

Is your feature request related to a problem? Please describe.
I want to get all securities from a screener, but there is a limitation in the number of securities we can get in each request.

Describe the solution you'd like
Adding a parameter offset to method Screener().get_screeners.

s.get_screeners(screen, count, offset)

I have the code. If you agree I will open a PR with this change.

[RudyNL]

@fabioDMFerreira A very useful extension. It would be better if count>250 can be applied instead of working with an offset and pagination. In case count>250 you can merge the results of the subsequent pages so the original call will work for any value of count.