fix(tagsInput): Set element's validity when options change

Set element's validity when minTags, maxTags and allowLeftoverText
options change. Other validation options don't need to be monitored
because they are only used upon some user interaction with the directive.

Closes #154
refactor(tagsInput): Add default values to all options

Add default values to all options so some parts of the code become a
bit simpler by not having to check for 'undefined' before using an option.
Introduce a MAX_SAFE_INTEGER constant to be used as an upper bound for
maxTags and maxLength options.
chore(build): Update build files [skip ci]