Update guidelines & add issues for new directive guideline

[andrewconnell]

New guideline for directives: if a public property / attribute on a directive takes a finite set of options, even if it is optional / default, the directive must:

• include an enum with the options
• validate the value of the attribute passed in is valid & if not, log an error in the console using $log.error()

This was recently implemented on the uif-icon directive... currently in the dev branch. See how it's done here:

• iconEnum.ts
• testing for valid value
• logging error

This will require the following

• add a new guideline to the directive guidelines doc
• create enhancement issues for existing directives... assigned to original author