Make behaviour of Escape key press configurable. (Issue #65)

This change implements the proposal in issue #65:
A modal can be closed by pressing the Escape key, if the `allowEscapeKey` argument is set to `true`. This currently is the default.

This changes the previous default behaviour. Closing by pressing Escape can now be configured independently from displaying a Cancel button.

As the new default for this configuration option is `true`, the idiom

    option = argument || default

is not longer a viable solution for the application of arguments and default values (consider the case of using a default value of `true` and wanting to override it with `false`). Thus, a helper function `argumentOrDefault` was implemented and for better visual consistency the application of all arguments and default values was changed to use this new function.