Non-default thousands separator

[t-bltg]

Add the ability to change the default thousands separator.

julia> using Formatting
julia> Formatting.THOUSANDS_SEPARATOR[] = ' ';
julia> format(1_000_000, commas=true)
"1 000 000"

No performance penalty observed.

Fix #45.

[ScottPJones]

I don't think adding global state is a good idea at all.
This can be added with a keyword argument to format, such as sep='_'

[kescobo]

This package is no longer maintained (see #111). Please consider using Format.jl instead, and if this feature is unavailable / still desired, open an issue or PR there.