Infer calls to str.format() on names

[jacobtylerwalls]

Future enhancement could infer this value instead of giving an empty string:

from astroid import extract_node
call = extract_node("""
x = 'python is {}'
x.format('helpful sometimes')
""")
call.inferred()[0].value  # gives ""

Originally posted by @jacobtylerwalls in #1602 (comment)