Allowing whitespace between key-value pairs of section parameters for nicer formatting.

In cases where there are too many parameters to a section to fit on a single line, it's much easier to format the parameters using newlines an indentations. An example similar to the one in our codebase:

```dustjs
{@define_mapping name="doc_types"
    blog           = "page"
    article        = "page"
    our-clients    = "page"
    html-document  = "page"
    blob           = "page"
    event-document = "event"
    meeting        = "event"
    lecture        = "event"
    landing-page   = "sidebar-page"
    ...
/}
    
```

Instead of:

```dustjs
{@define_mapping name="doc_types" blog="page" article="page" our-clients="page" html-document="page" blob="page" event-document="event" meeting="event" lecture="event" landing-page="sidebar-page" ... /}
```
This patch extends the `params` rule in `dust.pegjs` to allow additional whitespace before and after the `=` terminal.
It also includes a test case in `coreTests.js` that uses the new formatting.