Add a --format text option

[brandonrobertz]

This PR adds a --format text option to the CLI interface.

I use this regularly because pdfplumber has among the best layout preserving methods for PDFs, especially machine generated ones. Exposing the page output via CLI lets me use pdfplumber as a general purpose PDF-to-text tool.

Usage:
pdfplumber --format text file.pdf > file.txt

If there's some other way I should perform this (other than print(extract_text(...)) LMK and I'll make changes.

[jsvine]

Thanks! And glad to hear pdfplumber has been useful. I think this makes a lot of sense as a feature. Future iterations could allow passing relevant config params, such as --x-density, --y-density, --x-tolerance, --y-tolerance, et cetera. But even this config-less version seems worth including.