Add support to use premailer as fully command line program

This add the support to use premailer in the console by calling the main python module.

Also add all configurations as command line options.
```
$ python -m premailer -h
Usage: python -m premailer [options]

Options:
  -h, --help            show this help message and exit
  -f FILE, --file=FILE  Specifies the input file.  The default is stdin.
  -o FILE, --output=FILE
                        Specifies the output file.  The default is stdout.
  --base-url=BASE_URL   
  --remove-internal-links
  --exclude-pseudoclasses
  --remove-style-tags   
  --remove-star-selectors
  --remove-classes      
  --strip-important     
```

With support for stdin and stdout.

```
$ echo '<style>.heading { color:red; }</style><h1 class="heading"><a href="/">Title</a></h1>' | python -m premailer --base-url=http://google.com/ 
<html>
<head><style>.heading { color:red; }</style></head>
<body><h1 class="heading" style="color:red"><a href="http://google.com/">Title</a></h1></body>
</html>
```