[HTTPS-Everywhere] check extension of XML files

[Stephen Gildea]

I've been trying to update one of the rules files for HTTPS Everywhere,
and I eventually figured out that the reason I'm having problems is
that I'm editing in place in my Firefox profile extension directory,
and my editor is leaving behind backup files.

So far so good, but: HTTPS Everywhere is reading the backup files!

I propose that instead of reading *every* file in the content/rules
and HTTPSEverywhereUserRules directories, that the extension read only
files with names ending in ".xml" -- that will avoid backup files.

Meanwhile, I'll send you my rule file patch separately.

 < Stephen