This is similar to https://github.com/apple/swift/blob/master/CONTRIBUTING.md
with an explicit clause that allows changing license for #1073.

This is so that the format with a text_tyle will compile even if
header-only mode isn't enabled. Addresses #1063.

percentage.

This helps with compatibility with Python's format strings.

* Fix order of initializers

* Remove unused parameter

* Disable warning "-Wswitch-enum"

* Clarify usage of fmt::arg

Document that fmt::arg takes a non-owning
reference, even if that reference is to
a temporary. As such, users should make sure
the lifetime of the reference lasts as long
as the named argument.

* Clean up language

Remove mentions of `std::reference_wrapper` and rvalues
in favor of more common terminology like dangling references.