Support multiple named locales

[phensley]

Extends the compiler to support multiple named locales that can be selected by formatters.

In this example this assumes the compiler has been configured with a default locale and one named locale "user" set to es:

{t|datetime}
{t|datetime locale:user}

output:

February 10, 2020
10 de febrero de 2020

[CLAassistant]

All committers have signed the CLA.