Initial value and validators arguments missing for HashTagTextField

[christenbc]

Thanks for this package, it is amazing.

I was trying to figure out if there was an option to set initial value for the HashTagTextField but it looks like there is not option for that as there is for TextFormField. This is a pity, probably easy to add as a feature, very desirable in my opinion. So as the validator, it would also be great to have it as an argument.