Allow specifying custom input pattern #84

Hi, in the application that we currently use this library in, we use it to input codes with the regex pattern of /^[0-9a-zA-Z]{6}$, however, this library only allows enforcing of /^[0-9]+$/, otherwise all characters are allowed.

Would it be possible, or desirable, to allow the user to set their own regexp pattern? I could work on this as a pull request if needed.

Code example as to how I imagine this could look in use:

<code-input pattern="[0-9a-zA-Z]" ...></code-input>