Change localization interpolation

[mattesmohr]

This pull request experiments with changing the interpolation for localization.

String interpolation

Instead of passing interpolation arguments through the parameter...

Heading1("greeting.person", interpolation: "John Doe")

...you will now be able to use string interpolation directly.

Heading1("Hello \("John Doe")")

Format specifiers

Additionally the format specifiers have changed and now follows the "standard", so to say. While I find %st, %do, %in or %dt more straightforward, this change allows the reuse of existing string translations, maybe from an iOS app or similar.

"Hello %@" = "Hello %@";

So use %@ for string values, %lld for int values, and %f for floating point values.

Fallback

If someone mistakenly use the localization initializer without intending to use localization it returns the literal string instead of throwing an error, as requested in #148. The same applies if a string key cannot be found in one of the translation tables, although I leave it open for discussion whether it should return the string key in such cases instead.