logfetch.pl: save files as .txt rather than .html

[pdurbin]

The plain text format from http://irclog.perlgeek.de/crimsonfu has changed from HTML to text.

http://crimsonfu.github.io/irclogs/2013-05-16.html was the last log that's actually HTML. http://crimsonfu.github.io/irclogs/2013-05-17.html and later are actually plain text and are unreadable in a browser unless you do "view source."

To do:

• modify bin/logfetch.pl save files with the extension ".txt" rather than ".html"
• rename existing files from ".html" to ".txt"