make max-width of attribution control configurable

[jgravois]

in the majority of applications it makes sense to leverage as much horizontal space as possible to display attribution, primarily because of small form factor devices, but 55px is a little safer default because it accommodates a bottom oriented zoom control automatically.

since other possibly bottom aligned controls like L.Control.Scale and L.Controls.Layers don't have a static predictable width our best bet is to just expose a mechanism to let developers indicate just how much space should be reserved.

i _think_ its kosher to just tack on a new L.Map control constructor option and introspect for it ourselves later. if so, we'll need to find a new location in our own API reference to document it.

[jgravois]

@tomwayson reminded me that a generic L.esri.Options object would be a more appropriate place to stash a configurable setting than asking folks to start supplying custom constructor options to other API's objects.

the snippet below is an example of how a developer could customize the amount of space reserved for other controls.

// number of pixels to crop attribution width
L.esri.options.attributionWidthOffset = 200;
var map = L.map('map').setView([37.79, -122.45], 13);

technically, the proposed implementation could lead to unexpected behavior if a developer tried to pass unique values to different resizable maps as they were created (because the value will be different when map.on('resize'... is triggered later).

to be honest, i'm perfectly fine with that.

edit:
the code snippet has been updated now that tom's suggestion has been incorporated.

[tomwayson]

A little late on this, but LGTM! Only change I would have suggested is naming it L.esri.options w/ lower case "o" just cause it's not an object you can new up, but I ain't mad atcha.

[jgravois]

agreed. i'll push up soon.