feat(autocomplete): Down arrow hotkey option to open suggestion box if not shown

Add the down arrow event as a hotkey that opens the suggestion box if the
suggestion box is not visible. This feature is important because it gives
the user a method for browsing the list of tags without needing to filter.

Closes #54
Merge PR #67
refactor(autocomplete): Rename showSuggestionOnDownkey as loadOnDownArrow

Rename the showSuggestionOnDownkey option as loadOnDownArrow and perform
other improvements on the code.