Interpolated percentiles, markdown-tabular output format, CSV-quote preservation

Major features:

• Interpolated percentiles are now available using mlr stats1 -i or mlr merge-fields -i. Non-interpolated percentiles are the default. The former resemble R's type=7 quantiles and the latter resemble R's type=1 quantiles. See also http://johnkerl.org/miller/doc/reference.html#stats1 and http://johnkerl.org/miller/doc/reference.html#merge-fields.
• Markdown-tabular output format is now available using --omd: please see http://johnkerl.org/miller/doc/file-formats.html#Markdown_tabular and #106.
• For files using CSV input as well as CSV output, there is now a --quote-original option which outputs fields with quotes if they had them on input. The was-quoted flag isn't tracked on derived fields, e.g. if fields a and b were quoted on input, then in mlr put '$c = $a . $b the c field won't be quoted on output. As such, this option is most useful with mlr cut, mlr filter, etc. The use-case from the original feature request #77 (comment) is in trimming down a huge CSV file in order to facilitate subsequent in-memory processing using spreadsheet software.
• The cookbook at http://johnkerl.org/miller/doc/cookbook.html has been extended significantly.

Minor features:

• You can now set a MLR_CSV_DEFAULT_RS=lf environment variable if you're tired of always putting --rs lf arguments for your CSV files: http://johnkerl.org/miller/doc/file-formats.html#CSV/TSV/etc.
• The printn and eprintn commands for mlr put are identical to print and eprint except they don't print final newlines.
• It is now an error if boundvars in the same for-loop expression have duplicate names, e.g. for (a,a in $*) {...} results in the error message mlr: duplicate for-loop boundvars "a" and "a".
• The strptime function would announce an internal coding error on malformed format strings; now, it correctly points out the user-level error.

Bug fixes:

• Percentiles in merge-fields were not working. This was fixed; also, the lacking unit-test cases which would have caught this sooner have been filled in.
• Miller's CSV output-quoting was non-RFC-compliant: double-quotes within field names were not being duplicated. This has been fixed (#104).

Brew update: Homebrew/homebrew-core#2698