improving << parsing logic

[edublancas]

this is related to: #525

this works (spaces in both sides):

%sql df << select * from penguins.csv

but this doesn't:

%sql df<<select * from penguins.csv
%sql df    <<select * from penguins.csv
%sql df    <<    select * from penguins.csv

we should also cover the cases where there is a =:

%sql df=<<select * from penguins.csv
%sql df    =<<select * from penguins.csv
%sql df    =<<    select * from penguins.csv
%sql df=   <<    select * from penguins.csv

[edublancas]

hey @anupam-tiwari FYI: we merged #609 which allows more flexible spacing when using = <<, so for this issue, the only remaining part is then having <<