Add a public method to focus the component

[kiasyn]

Fixes #150

[chrissainty]

This looks great @kiasyn. Just need a couple of things before I can merge it though:

1. Could you update the Blazor Server sample with the example as well
2. Could you add a some details to the readme so people know this feature exists and how to use it

Once these bit are done, this can be merged straight away

[zippo-lighter]

@chrissainty what is blocking the merge into the main repository?

[chrissainty]

@zippo-lighter As per the comment directly above yours states, there is no documentation or samples for this work.