add locale support for name specifiers

[lestrrat]

• add WithLocale(Locale) to localize name specifiers %A %a %B %b %h %p; numeric specifiers unaffected
• Locale is an interface built via NewLocale(...) functional options; English default derived from stdlib time, empty entries fall back to English
• inflected languages (format vs stand-alone month forms) handled by compiling one Strftime per context; a future %OB/%Ob can be added as an optional interface without breaking the API

Note: this ships no actual localizations — the library carries no per-language name data. It provides the escape hatch so users can supply their own names (or implement Locale themselves, backed by a map / computed values / an external dataset).

Closes #64