add strip_encoding_cookie transformer

closes #3245
test strip_encoding_cookie transformer
fix typo in %load docstring

Since I was reading it for reference
Merge pull request #3662 from minrk/coding_transform

add strip_encoding_cookie transformer

strips # coding: utf-8 comments from input, which can cause SyntaxErrors

closes #3245
test that cell magic preempts other transformers
push cell magic to the head of the transformer line

I'm not 100% sure this is the right fix (ping @takluyver for review),
since it is unclear what the physical/logical distinction is,

but this does allow the cell magic transform to preempt all other transforms.

closes #3604
don't close string from inside re.VERBOSE comment

some highlighters get confused :(
don't allow cell magics mid-cell
move cell magic after prompt transformers
update cell magic transform test
show UsageError for cell magics mid-cell
add InteractiveShell.show_usage_error

for showing UsageError,
rather than explicitly calling `showtraceback()`
to render an exception that wants no traceback.
rollback UsageError for cell magic mid-cell

regular SyntaxError is raised