RFC: add ENV for locale

[wlbksy]

add ENV["JULIA_LOCALE"], so we could make use of ENV
i18n helpdb is put byside of original helpdb.jl

$ ls usr/share/julia/
base  doc  examples  extras  helpdb.jl  helpdb_zh_CN.jl  test  ui

julia> ENV["JULIA_LOCALE"]="zh_CN"
"zh_CN"

julia> help(sin)
Loading help data...
Base.sin(x)

   计算 "x" 的正弦值，其中 "x" 的单位为弧度。 # This is Chinese ... 

julia> Base.locale("")

julia> help(sin)
Loading help data...
Base.sin(x)

   Compute sine of "x", where "x" is in radians

julia> Base.locale("zh_CN")

julia> help(sin)
Loading help data...
Base.sin(x)

   计算 "x" 的正弦值，其中 "x" 的单位为弧度。

julia>

[nolta]

This isn't necessary. There already exist standard environment variables for setting the locale (e.g., LANG) and functions for determining the current locale (e.g., setlocale).