Add support for decoding into structs with a custom tag

[gastrodon]

I'd like a feature for a project that I'm working on to be able to do something like gocty.FromCtyValue, but for a struct with field tags of a kind other than cty:"". Is this within the scope of this project / compatible with design goals?

I've taken a stab at it here - that branch updates func structTagIndices to take a tag string instead of always using cty, and adds a new function func FromCtyValueTagged that takes a new parameter tag string

My project uses this function to decode values into structs that are provided by a plugin and everything seems to work ( +cty tests pass )

[gastrodon]

pr'd #181