FEATURE: Add Icon and Header to Off Canvas

[deotte]

Issue

#240

Problem

Currently, the offCanvasComponent does not support the ability to add a custom header to the top. The only way you can render the offCanvasComponent is with the default header having a large close button at the top.

Solution

Added another binding to the goOffCanvasService.openOffCanvas call to be able to pass in a header. Because of the interesting way that the goOffCanvasComponent is rendered from the goOffCanvasService, I could not directly add an input to the component itself.

Before

After

[grahamhency]

@deotte thanks for taking the initiative on this one. I've got a few changes and comments. The only thing missing here is updated documentation in the go-style-guide project. (See off-canvas-docs.component.html)

You can start the style guide project with npm run style_guide

[grahamhency]

@deotte this looks good to me! Thanks for making those changes.