Add text support for extensions. Thanks, @sn4k3

[SKProCH]

Rework of #38

Thanks, @sn4k3

[sn4k3]

Thanks, I will test :)

Minor stuff, but should [ConstructorArgument("textfirst")] follow the rule of capital [ConstructorArgument("textFirst")] ?

Also readme should be fixed: <Button Content="{materialIcons:MaterialIconExt Kind=Play, Text=Play}" /> to <Button Content="{materialIcons:MaterialIconTextExt Kind=Play, Text=Play}" />

[SKProCH]

Yeah, i missed that. Gonna fix tomorrow