Resolution to issue #23

This change allows developers the opportunity to provide their own help formatter_class.  This allows for flexibility enough that maybe enough to resolve issue #23. 

For example:

```
import begin
import argparse
@begin.start(formatter_class=argparse.RawTextHelpFormatter)
def main():
    """Plain text formatting for this help:
    - how does it look?
    Params:
        foo is yummy... 
    """
```

Yields:
```
usage: pos.py [-h]

Plain text formatting for this help:
    - how does it look?
    Params:
        foo is yummy...


optional arguments:
  -h, --help  show this help message and exit
```
Merge pull request #31 from mjdorma/patch-3

Customise help formatter class

An argparse help formatter may be based to the begin.start decorator to replace the default argparse help formatter.