Add function to parse Cap from string.

[r10r]

Hi,

I've added a function to parse a string intoCap.
I use it to look up capabilities by it's string value. e.g:

func (rt *Runtime) hasCapability(s string) bool {
    c, exist := capability.Parse(s)
    if !exist {
        rt.Log.Warn().Msgf("undefined capability %q", s)
        return false
    }   
    return rt.caps.Get(capability.EFFECTIVE, c)
}

It would be nice if you could merge this. Thanks!