Deprecate wb-blank, fix formatting issues, bug fixes

This addresses one big open issue and several other small ones:

Merge the functionality of wb-blank into wb-fillable (closes #17)

• Some refactoring to turn chunks of code into reusable functions, especially where code was repeated
• Blank sheet can now be produced with --blank flag instead of calling a separate script
• Revise documentation to reflect this change
• Delete wb-blank and related code

Formatting issues with Excel output from wb-fillable (closes #27)

There were two related issues with Excel output:

• When fields had blank strings as their description in WB_FIELDS_ORDERED_WITH_DESCRIPTION, wb-fillable output a spreadsheet where those values were missing, so (for example) the description for title spanned the columns for field_metadata_title, id, parent_id, and other surrounding columns
• Description text for the final column went off the edge of the page because it was being centered within multiple cells

These were both fixed by changing 'center_across': True (which centers text within all the blank cells around a given cell) to 'align': 'center' (which centers within a single cell).

While troubleshooting, I also swapped in the variable BLANK_DESCRIPTION for '' in default_specs, so if we want to in the future, we can put something like "---" instead of blank space for fields with no descriptions. (This solved the first formatting issue but not the second.)

Miscellaneous small fixes from open issues

• Change field_linked_agent_ROLE to field_linked_agent_RELATOR (closes #24)
• Add ability to count PDF pages and fill to field_extent (closes #23)
• Change --filefolder flag to --files (closes #25)
• Move logic for constructing output file names into extract_file (this should close #15 unless there's something more we want to do there)
• Get rid of some print statements that duplicate info now given in command line help text

Update install instructions

This release updates the installation instructions to reflect the publication of the package to PyPI.

Initial publication!

We are publishing this package for the first time!