luci-base: add support for plural translations and contexts in Lua api

luci-base: add support for plural translations and contexts in Lua api

 - Introduce a new luci.template.parser.ntranslate() function which
   takes a count, a singular and a plural translation string as well
   as an optional context argument and returns the appropriate,
   language specific plural translation.

 - Introduce an optional translation context argument in the existing
   luci.template.parser.translate() function

 - Support translation contexts in LuCI template directives.
   Translation messages are split on the first unescaped pipe
   character and the reamining string after the pipe is treated
   as context.

Examples:

 - `string.format(p.ntranslate(n, "1 apple", "%d apples"), n)` will
   return an appropriate plural translation for the given amount.

 - `translate("Load", "The system load")` will return an appropiate
   translation for `Load`, using `The system load` as disambiguation
   context (a `msgctxt` directive in *.po files).

 - Likewise `<%:Load|The system load%>` will translate the word
   `Load` while using the remainder of the string as context.

 - To use pipes in translations strings literally, they must be
   escaped: `<%:Use the "\|" character%>` will translate the literal
   string `Use the "|" character`.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>