Better environment-variable support, new 'count' verb, bugfixes

Features

• The new count verb is a keystroke-saver for stats1 -a count -f {some field name}.
• --jsonx and --ojsonx are keystroke-savers for --json --jvstack and --ojson --jvstack, which is to say, multi-line pretty-printed JSON format.
• The new -s name=value feature for mlr put and mlr filter gives you simpler access to environment variables in your Miller script, as requested in #315.

Bugfixes

• mlr format-values is no longer SEGVing on CSV/TSV input. This was reported on #330.
• #313 fixes a corner case when field names within command-line arguments have embedded newlines.
• Line/column indicators for JSON-formatting error messages are now correct (previously they were showing up as 0).
• end {print NF} no longer SEGVs. This was reported in #330.
• Several broken doc links were fixed up as reported on #329.

Windows note

• The AppVeyor build has been broken for a while so there is no Windows executable attached to this release -- when I fix that there will be a 5.8.1 with Windows binaries. My apologies for the delay. Issue #354 is open to track this.