Add user options and commands to customize wrap width value

By default, Wrap Plus use Sublime Text settings `wrap_width` or `rulers` to know at which column to wrap text.

In some cases, it might be useful to define a custom width to wrap a specific file. I've added some options to do that:
- `WrapPlus.wrap_width`: a int to define the wrap width size, instead of using values from `wrap_width` and `rulers`, eq: `"WrapPlus.wrap_width": 80,`
- `WrapPlus.wrap_width_custom_list`: a list of predefined width size, to quickly switch to one of them using Command Palette, eg: `"WrapPlus.wrap_width_custom_list": ["72", "80", "100", "120"]`

I've also added a Command Palette entry which can be used in 3 different ways:
- very quickly change the current wrap width size to a fixed one, eq:

```
"command": "wrapplus_setwidth", "args": {"width": "72"}
```

- change the current wrap width size to one of the predefined width from `WrapPlus.wrap_width_custom_list` setting:

```
"command": "wrapplus_setwidth", "args": {"source_list": "wrap_width_custom_list"}
```

- change the current wrap width size to one of the `rulers` setting:

```
"command": "wrapplus_setwidth", "args": {"source_list": "rulers"}
```


All those command can be placed in all Sublime Text file (`*.sublime-commands`, `*.sublime-keymap`, `*.sublime-menu`)