Add full support for optional positional paramaters.

If a positional parameter is optional the default value will be used if an underscore is passed.

so for a command defined as such

```
name: Print
description: Echos back what you type.
arguments:
- name: text
argtype: quoted text
optional: true
defaultValue: Much default wow.
position: 1
- name: effect
argtype: enum
options: allcaps, camelcase, leetspeak, shouting
position: 2
```
the following command should be equivalent

```
/print "Much default wow." allcaps
/print _ allcaps
```

and properly implemented would return ```MUCH DEFAULT WOW.```