Format dictionary for key string eval

ENH: Added optional flag 'strip_chars' to remove POSIX characters '-', '<', '>' from docopt.docopt dictionary.  Operations that evaluate the key strings will not throw a syntax error and **kwargs syntax is preserved.

```python
"""
Usage:
  my_test --work <time>
  my_test --lazy

Options:
  --work boolean
"""
import docopt

def my_lazy(lazy=False, **kwargs):  # BAD: (--lazy=False, **kwargs)
    if lazy:
        print "Taking a break..."

def my_function(work=False, time, **kwargs):  # BAD: (--work=False, <time>, **kwargs)
    if work:
        for t in time:
             print "Doing some work!"
        my_lazy(**kwargs)


if name == "main":
    kwargs = docopt.docopt(doc, strip_chars=True)
    # Application-specific dispatch logic
    if kwargs['work']:
        my_function(**kwargs)
    else:
        my_lazy(**kwargs)
```