Added support to brazilian portuguese, with the ability to support other languages

I created a new file and a new package, and put all the alists with the regexes in there, and created a simple API to change between languages:
```lisp
(set-lang! :pt_BR)
;=> :PT_BR
(singular-of "países")
;=> "país"
(curr-lang)
;=> :PT_BR
(available-langs)
;=> (:EN_US :PT_BR)
```
I also implemented the equivalent tests for the portuguese language. Maybe some tests of changing the language and adding new vocabulary should be added. I think I'll do that soon.

My goal was creating a simple structure to add support for new languages when needed without much trouble (and without changing much of the existing code, too). To add a new language, one needs to create the 4 special variables (`*plurals-LANG*`, `*singulars-LANG*`, `*uncountables-LANG*` and `*irregulars-LANG*`) and add an entry in the `*langs-plist*` variable.

All this needs to be added in the README.md.