Make rolling file names more configurable

There are two changes in this PR that address how rolled file names are generated. The first allows the "rolling" part - either number or date - to be placed between the basename and extension, or after the basename and extension (really you can put it anywhere now). The second allows the date to be customized. These two changes address #35 and #73 

For rolling files by number, the new syntax would be `"filename{{.%d}}.ext"` to emulate the current behavior or `"filename.ext{{.%d}}"` if you want the number to appear at the end. This is what you would use as the `:filename` option.

For rolling files by date, the new syntax would be `"filename{{.%Y%m%d}}.ext" to emulate the current behavior. If you wanted your date format to be ISO8601 compliant you would use `"filename{{.%Y-%m-%dT%H:%M:%S}}.ext"`. You could also put the roller at the end, too, just as with the number roller above.

Full on config example ...

```ruby
Logging.appenders.rolling_file( 'roly-poly',
  :filename => '/var/log/roly-poly-app{{.%Y-%m-%d}}.log',
  :age => 'daily',
  :keep => 7
)
```

@slyphon and @javmorin, would you be willing to take a look at this code and comment if this will meet your needs or not? Thanks a bundle!