Add a simple query system

- Extend clint.textui.prompt with a query function.
- Add clint.textui.validators to validate the user input.
- Add a prompt example showing different usages.
- Update the README to demo the query function.

examples/prompt.py

@@ -0,0 +1,21 @@
+#!/usr/bin/env python
+# -*- coding: utf-8 -*-
+
+import sys
+import os
+
+sys.path.insert(0, os.path.abspath('..'))
+
+from clint.textui import prompt, puts, colored, validators
+
+if __name__ == '__main__':
+    # Standard non-empty input
+    name = prompt.query("What's your name?")
+
+    # Set validators to an empty list for an optional input
+    language = prompt.query("Your favorite tool (optional)?", validators=[])
+
+    # Use a default value and a validator
+    path = prompt.query('Installation Path', default='/usr/local/bin/', validators=[validators.PathValidator()])
+
+    puts(colored.blue('Hi {0}. Install {1} to {2}'.format(name, language or 'nothing', path)))