various minor improvements

- use random.choice() instead of random.randrange() and checking for each number
- use proper `not` operator instead of `== False`
- use format strings instead of manually concatenating the components of the post
- use getpass.getpass() instead of input() for entering the user's password
- make the script executable using a shebang and chmod +x
- gitignore .lgfbot/
- use sets instead of lists for static containment checks

.gitignore

@@ -0,0 +1 @@
+.lgfbot