Roundtrip printing

[YingboMa]

Currently, show displays units that are cannot be copy-pasted into Julia REPL. This makes serializing expressions impossible if the expression contains unitful objects.

julia> using Unitful

julia> unit = u"m/s";

julia> show(unit)
m s⁻¹
julia> :(x = $unit)
:(x = m s⁻¹)

In Julia, by convention, pretty print should only be done on MIME methods, and default show should do roundtrip

[giordano]

In Julia, by convention, pretty print should only be done on MIME methods, and default show should do roundtrip

Really? I thought that was repr (not that the result here is different)

[giordano]

Duplicate of #391