Add optional requireThisBinding in named-functions-in-promises rule

[michalsnik]

As discussed in #64 we agreed that it might have a sense to add new optional setting, called for example requireThisBinding which would require using current module's methods instead of custom ones.

Example with requireThisBinding = true:

// Good
user.save().then(this._reloadPage.bind(this));

// Good
user.save().then(() => this._reloadPage());

// Bad
user.save().then(reloadPage.bind(this));

// Bad
user.save().then(() => reloadPage());

Default setting would be false so all of the above examples will work fine and we'll keep it backwards compatible.

I don't have any better name for the setting in my mind at this moment, so feel free to add your propositions in comments.