Add CLI mode

[njourdane]

Add ability to execute slugify through command-line.

Usage

Typical

$slugify -s 'some string'
some-string

Via stdin

$echo 'some string' | slugify
some-string

With options

$slugify -s 'Some String' -l
some-string
$slugify --string 'Some String' --lower --replace '_'
some_string

Help

$slugify -h                    
Usage: slugify [options] -s 'string'
       echo 'string' | slugify [options]

Options:
  -s, --string            the string to slugify
  -r, --replacement char  defines words separator
  -R, --remove regex      remove characters
  -l, --lower             result in lower case
  -h, --help              display this help and exit