Add "custom" output for CLI

[Neamar]

Let the user choose its own templating scheme on the bunyan CLI.

Sample use:

... | bunyan -o custom --template "{{username}} {{error}}"

Arbitrary code can be injected:

... | bunyan -o custom --template "{{username}} {{error.toUpperCase()}}"

And colors are handled (not in the cleanest way) with _cS and _cE:

... | bunyan -o custom --template "{{ _cS('grey') }}{{ req.method }} {{ req.user || '???' }} {{ req.url }}{{ _cE('grey') }}"

(this use the default stylize function, and will revert to no-color on non ansi terminals)

Eventually, we can get to quite complex templates -- here is the one i'm using:

{{ _cS('grey') }}{{ req.method }} {{ req.user || '???' }} {{ req.url }}{{ _cE('grey') }} {{ _cS(res.status > 500 ? 'red' : res.status > 400 ? 'yellow' : res.status > 300 ? 'cyan' : 'green')}}{{ res.status }}{{ _cE('green') }} {{ _cS(res.time > 800 ? 'bold': 'grey')}}{{ res.time }}ms{{ _cE(res.time > 800 ? 'bold': 'grey') }} {{ _cS('underline') }}{{ res.restCode }}{{ _cE('underline')}} {{ msg }}

For the following result:

Add an optional npm dependency (underscore) and use the built-in _.template.

[nblasgen]

If I were to update this pull request, would it be accepted? I'm in need of any other kind of console display than the default options, and I prefer this implementation (though via a config file rather than a very long string) than writing multiple static output modes.