Support Locale for date parsing in sprintf format

When parsing a date with sprintf, we use by default the local Locale.

For example:

 mutate {
    replace => { "message" => "%{start}[%{+dd/MMM/YYYY:HH:mm:ss Z}]%{end}" }
  }

could produce something different when running on a french computer than on an english computer: 14/déc./2014:00:13:42 +0000 vs 14/Dec/2014:00:13:42 +0000.

It could be nice to specify for a date the Locale we want to use whatever is set on the computer.

%{+LOCALE@DATE_FORMAT} such as:

 mutate {
    replace => { "message" => "%{start}[%{+fr@dd/MMM/YYYY:HH:mm:ss Z}]%{end}" }
  }