Add support for custom Octicons in admin nav sections

[wosevision]

This pull request adds support for the ability to specify custom icon classes to associate with Admin UI nav sections, i.e. those configured by keystone.set('nav', ...).

I proposed this feature not too long ago but didn't realize then what an easy change it would be (the feature is basically already "built in" and just needed a tweak to "enable" it).

To use this feature, instead of writing

keystone.set('nav', {
    'small animals': ['mice', 'rats']
});

you can instead write

keystone.set('nav', {
    'small animals': {
        lists: ['mice', 'rats'],
        icon: 'octicon octicon-squirrel'
});

which will override the default 'bullet' icon. All previous behaviours are left intact (bullet is still default if no icon provided, component still "auto-selects" an icon based on list name).

The auto-selecting behaviour struck me as a bit presumptuous; this pull request hopes to address that by opening it up completely.

Might deal with #3505.

[kmiddlesworth]

Hey there, I'd love to know if a merge is planned for this. This functionality would be great!

[DaTebe]

I would also like to see this feature

[Norbz]

Once again, a small but usefull feature dead in its tracks because of no one validating PR...