Add a few warnings, expand_tabs un-hardcode tab_size

[jdmonin]

Hello,

Thanks for making txt2pdf, it's very useful and easy to understand!

To make it easier to use for a first-time user (like me), I added these warnings:

• If text wrapped, print the max width seen to help determine what parameters to change
• Warn if unexpected tab chars when not using --tab-size or --tab-replacement

And fixes 1 small thing I saw:

• expand_tabs: un-hardcode tab_size 4 in %

Tested in python 3.8 and 2.7 (ubuntu 20.04).

Please merge if this is useful, or let me know what else I need to do for this PR. Thank you, -Jeremy