Add an 'unexecuted' cell (with missing prompt_number) to test.ipynb.

This makes some tests fail because they assume that each cell has a
'prompt_number' attribute.
Quick fix for failing tests. This will be cleaned up soon.

This is only a quick fix to make the tests pass. Note that some tests
which access the 'prompt_number' attribute were not affected by the
additional unexecuted cell in tests/test.ipynb which was added in
commit ec19b407dee30fb7e692ce753af9fc5bd3808f8f. So this probably
indicates that not all corner cases are covered.
Replace ad-hoc/broken code with safe method to extract the prompt_number of a cell.

The helper function returns the converter's blank symbol ('&nbsp;' for HTML,
space for other converters) if the cell doesn't have a prompt_number.
PEP8-ify several files
Unused imports.
PEP8-ify rest of the repository.
LaTeX converter: command line args for preamble filename and to exclude certain cell types
LaTeX converter: remove problematic characters from filenames
Choose the best format available for display_data and only show that one
Options to exclude certain sections (source code or cell output) from exported result
Store clean_filename.
Standard-compliant docstring for clean_filename.
Clearer/simpler code suggested by @fperez.

https://github.com/ipython/nbconvert/pull/43#discussion_r2114903
Added a test for clean_filename.
update ref pyfile
update ref texfile
Fixed weird ImportError introduced during rebase.
update ref rstfile
update ref markdownfile
Merge pull request #1 from Carreau/pr52

Fix nbconvert Pr52