Allow using a Parser as a formatter

[ymost]

I think it would nice and elegant if the same object could be used both as a formatter and a parser. This can be achieved by adding a format method:

>>> p = compile("It's {}, I love it!")
>>> p.parse("It's spam, I love it!")
<Result ('spam',) {}>
>>> p.format('spam')    # <----  This is the new suggested method
"It's spam, I love it!"

Granted, one could simply hold on to the original string and use that, but I think it's useful if we only needed the Parser object for both of these. For example, if you pass the Parser as an argument to a function, currently you need to pass the format string as well, otherwise you have to access the internal _format field.
At the very least, we should have a get_format method to retrieve the internal _format field.

Another reason is that if we are using two different objects for formatting and for parsing, it increases the chance that someone creates a bug by changing one of the objects and forgetting to change the other one.

[wimglenn]

Similar was suggested in #100

[ymost]

I think what I'm suggesting is much simpler. It could hardly be considered new functionality, just exposing existing functionality. Basically what I have in mind is:

def format(self, *args, **kwargs) -> str:
    return self._format.format(*args, **kwargs)

Does this seem reasonable?

[wimglenn]

It seems reasonable, one thing does occur to me though that the Parser supports some "extra" types which do not make sense for a formatter, e.g.

>>> p = compile("{:w}")
>>> r = p.parse("xyz")
>>> p._format.format("abc")
ValueError: Unknown format code 'w' for object of type 'str'

So the format method may be more difficult to implement in a full-featured way, rather than just exposing ._format.format().

Looking at the Parser class, I also don't see any obvious reason why the ._format attr is internal, perhaps a (read-only) property could just be added to return the format template string, instead of a format method?

class Parser(object):
    ...

    @property
    def format(self):
        return self._format

[ymost]

Wow, I didn't know about the extra types. I guess parse() is not exactly the opposite of format()...
Anyway a read-only property seems good!