Support locale-specific formatting of date/time

[lyricnz]

On chart X-axis labels and last_updated column. See #188

[lyricnz]

This code obeys $LC_TIME first, then the (platform-specific) locales second - see https://github.com/jeandeaual/go-locale
(I have submitted upstream issue to use LC_xxx variables - jeandeaual/go-locale#10)

The actual formatting uses https://github.com/goodsign/monday to translate day/month names (including abbreviations).

[lyricnz]

Further work:

• use local-specific formats (not just translations) doesn't seem to be good alignment
• change Numericf() and Monetaryf() to use the same locale-detection (with $LC_NUMERIC and $LC_MONETARY overrides)

[lyricnz]

Example LC_TIME=hu-HU ./bin/cointop

[miguelmota]

Thanks @lyricnz! Merged