Avoid duplicated Custom Element name registration errors

Avoid duplicated Custom Element name registration errors - thank you @kleingtm (#139).

Add 'vueInstanceCreatedCallback' that is triggered after creating Vue instance

Add 'vueInstanceCreatedCallback' to options, that is triggered after creating Vue instance.

fix(#128): Empty boolean prop not recognised

fix(#128): Empty boolean prop not recognised, thanks @woubuc

Fix "Throws error when component use mixin - Typescript"

fix(#125): Throws error when component use mixin - Typescript, thanks @akki-jat

Expose Vue instance using `getVueInstance()` method

Expose Vue instance using getVueInstance() method.

Thanks @allenhwkim (#123)

Allow template syntax for slots

Merge "Allow template syntax for slots" (#121) PR. Thanks @dennisbaskin!

TypeScript fixes

TypeScript fixes - #115, thanks @akki-jat

fix(#113): Getting "propsValue.match is not a function" by setting property with querySelector

Fixed #113, thanks @creynir!

Bug Fix for Boolean Prop Validation

Bug Fix for Boolean Prop Validation (#103). Thanks @auzmartist !

Fix Error Using Multiple Prop Types

Fix error using multiple prop types, thanks @dmlinn!