Added walls for blocking exits when using room coordinates

[ProjectStickman]

Issue: When using rooms with coordinates all rooms get connected automatically.

Example: When you have the room
1 2
3 4

and you don't want the player to directly move from 1 to 3 there is no way to block the movement permanently (so it doesn't even show up in look, move etc.).

Solution: Adding a "walls" parameter to rooms which allows you to define a direction in which no exit will be generated. The walls won't show up in look or move unlike doors, causing less confusion and a cleaner syntax.

YAML Syntax:

walls Array
Walls permanently restricting access from this room. Each walls entry has the following field:

direction string
required Direction which gets blocked by the wall.

[shawncplus]

Welcome to the project, appreciate the PR. There was a bunch of discussion about this I think last year some time an I think most people settled on using metadata for this as it didn't require modifying core and accomplished the same result. Did you run into an issue using metadata?

[azigler]

Similar to #113. I discussed the pros and cons of this type of approach there. I agree with @shawncplus that this can be achieved without modifying core.