Support for Custom Slots added

[Lensy2016]

By using a new 'types' map when defining an intent it's possible to request the set of custom slot values for a given value. Without the types map the original behavior is maintained.

A new example is also added that demonstrates this behaviour

I'm not a python coder, so there may be tidier ways of doing bits of this! However it does all seem to work and has been tested by driving the example code in various 'unhappy path' ways.

[mattrossman]

I am surprised that this functionality is not already implemented. I don't want to check for all possible synonyms of a data value in my application, I want to get back a single ID corresponding to each data value. Thanks for adding this.

[ludusrusso]

I'm also interested in this feature!