[ToDo] Use forthcoming strings.ToValidUTF8

[ymmt2005]

golang/go#25805

This library uses string([]rune(s)) technique to remove invalid Unicode characters
in multiple places. They should be replaced with strings.ToValidUTF8 which will
be added in Go 1.13.

$ git grep '\[\]rune'
json.go:                        t = string([]rune(t))
json.go:                        s = string([]rune(s))
logfmt.go:                      t = string([]rune(t))
logfmt.go:                      s = string([]rune(s))
plain.go:                       t = string([]rune(t))
plain.go:                       s = string([]rune(s))

[ymmt2005]

strings.ToValidUTF8 will be added to Go 1.13
golang/go@3259bc4

[Hsn723]

Closing as this was implemented in #26 and released as v1.6.0