support for variables in multi-line statements

[Lucaszw]

Closes #80 , #79

Although line_magic automatically replaces variables, cell_magic does not.

query = "name = 'bob'"

%%sql
select count(1) from users where {query}

To escape use multi-brackets:

%%sql
select '{{I'm in brackets}}'

[catherinedevlin]

I... I... I did not even know that {} variable substitution worked. I never wrote that in! It must be bubbling up from, um, somewhere.

But OK, if it works in one place it should definitely work in all places! Thank you!