Supply IFormatProvider to CompactJsonFormatter/RenderedCompactJsonFormatter

[nblumhardt]

Message template rendering is culture-specific; the constructors for the included formatter types should accept an optional IFormatProvider (likely through an overload) so that consumers can opt into the locale of their choosing.

Note that v2 dev currently uses CultureInfo.InvariantCulture inline in the source code, via #54, which is a breaking change (the old default would use the current locale).

When this change is implemented, we should revert that breakage and again default to the current culture.