Add a show method for pretty printing of Decimals

[amellnik]

Currently there's not a Base.show method for the Decimal type. This makes them hard to read on the repl, in notebooks, and in DataFrames.

[amellnik]

If people are attached to the verbose default show method this could be restricted to the text/html MIME type.

[Keno]

My suggestion here would be to have dec"1.0" string macro and use that for the default printing. Additionally, we can use :typeinfo to omit the wrapper in appropriate typed contexts.

[waldyrious]

Just to make things explicit, here's what this PR does:

julia> using Decimals

julia> # Current output
julia> Decimal(0.1)
Decimal(0, 1, -1)

julia> # New output with the code introduced in this PR
julia> Base.show(io::IO, d::Decimal) = print(io, string(d))
julia> Decimal(0.1)
0.1