Bring back color printing

[fredrikekre]

also remove an old takebuf_str method.

[fredrikekre]

Unrelated nightly windows failure, requires admin to merge.

[mortenpi]

Definite 👍 to removing takebuf_str. But what's the actual behavior difference with the colors? As far as I can tell, julia --color=yes docs/make.jl has color and julia docs/make.jl doesn't, both with this PR and current Documenter master, across Julia versions.

[mortenpi]

Ah OK, the difference is on Travis. The nightly builds still have less color than the 0.6 ones though, even with this PR, but that makes sense since this only touches the doctest printout. But why is Travis colorless with the nightly in the first place, even though Pkg.test() gets called with --color=yes?

[fredrikekre]

See JuliaLang/julia#25067, color printing is now governed by get(io, :color, false) rather than Base.have_color and in the case for this PR we print to an IOBuffer without color.