Add chaining of env 'add' methods

[robgraeber]

As discussed in #485, adds chaining for env 'add' methods like addGlobal, addFilter, addBlock, etc. I also wrote a test for addGlobal chaining and verified all the tests still pass. Let me know if you want anything improved.

@devoidfury @jlongster @carljm

This would make chaining like this possible:

Nunjucks.configure(templateDir, {
    autoescape: true,
    trimBlocks: true,
    lstripBlocks: true,
    express: app
}).addGlobal('func1', function(arg) {
    return 'asdf'
}).addGlobal('func2', function(arg) {
    return 'asdf'
}).addGlobal('func3', function(arg) {
    return 'asdf'
}).addGlobal('func4', function(arg) {
    return 'asdf'
});

[carljm]

Thanks @robgraeber ! Mind adding an entry to the changelog for this? Also, do you think this is worth mentioning in the documentation for those methods?

[robgraeber]

Hey @carljm sure, updated the changelog and made references in the docs. I think mentioning those functions have a return value now could be nice.

[carljm]

Merged, thanks for the contribution!