Windows' native terminal, `conhost.exe`, lacks font fallback support.
This causes some of the characters used in `wttr.in`'s terminal output
to not be displayed at all - placeholders are supplied in their place.
This PR provides a mode that, when enabled, translates missing glyphs
to ones available on every platform without (*I believe*) making any
compromises regarding their meaning; see translation table below.
| Character | C-UCP | Replacements | R-UCPs |
| :--------------- | :----: | :------------------------------------ | :----: |
| NORTH WEST ARROW | U+2196 | BOX DRAWINGS LIGHT ARC DOWN AND LEFT | U+256E |
| NORTH EAST ARROW | U+2197 | BOX DRAWINGS LIGHT ARC DOWN AND RIGHT | U+256D |
| SOUTH EAST ARROW | U+2198 | BOX DRAWINGS LIGHT ARC UP AND RIGHT | U+2570 |
| SOUTH WEST ARROW | U+2199 | BOX DRAWINGS LIGHT ARC UP AND LEFT | U+256F |
| HIGH VOLTAGE SIGN | U+26A1 | BOX DRAWINGS LIGHT DOWN AND RIGHT + UP AND LEFT | U+250C + U+2518 |
The [ftp](http://man.openbsd.org/ftp)(1) command is the default tool to fetch files over HTTP on OpenBSD. It could be nice to add it to the PLAIN_TEXT_AGENTS list so installing curl or wget is not necessary.
Its user-agent is, as described in the man page, “OpenBSD ftp”.