Resolution to Issue #20, Partial Resolution to Issue #4 and new toggle functionality

[jodyfarnden]

This version adds the ability to add custom svg icons to the buttons instead of being restricted to material icons. This should assist in resolution of #4. To use a custom icon it must be included in the project assets and the path and a custom name must be supplied to the button (parent menu button or child buttons will work)

The lowercase text css has been split into it's own class and a "toLower" input has been supplied to add the lowercase text styling if required. by default this is false and text appears normal. This should resolve #20.

An additional input "allowToggle" has been added - this allows the automatic retraction of the menu to be stopped if required. An example is if you are using the menu for an edit/save scenario, the save button press will not Toggle the menu if you set the allowToggle to false when validation fails.

Note that I have been unable to build the original project, I am testing the modified code in a new Angular 6 project and has been working, but may want to verify it still builds for the targeted version of angular.