Displays label properly on <SLDSInput> when both label and assistiveText are defined

[iowillhoit]

Fixes #314 and adds tests for

Adds a new warning in utilities/warning that helps prevent defining unneeded props.

[tweettypography]

Looks great to me! @donnieberg, do you want to review and merge this?

[donnieberg]

Looks great! I like this format of when the label is visible, use label and then when the label should be hidden, use assistiveText. Has this been added to the documentation? I'll merge this in but let's definitely add it there if it's not already :)