Actually overload Base's escape_string

While part of the code brought over from Base (or rather a PR to Base)
in 745385d, it was missed when adding the relevant imports to
overload the Base methods.

The main impact of this was slow 3-arg show performance, as raised in
issue #72.

Author: tecosaur

src/io.jl

@@ -261,7 +261,7 @@ Base.print(io::IO, s::Union{<:AnnotatedString, SubString{<:AnnotatedString}}) =
 Base.print(io::Base.AnnotatedIOBuffer, s::Union{<:AnnotatedString, SubString{<:AnnotatedString}}) =
     (write(io, s); nothing)
 
-escape_string(io::IO, s::Union{<:AnnotatedString, SubString{<:AnnotatedString}},
+Base.escape_string(io::IO, s::Union{<:AnnotatedString, SubString{<:AnnotatedString}},
               esc = ""; keep = ()) =
     (_ansi_writer(io, s, (io, s) -> escape_string(io, s, esc; keep)); nothing)