System calls / external commands, ASV/USV support, and bulk numeric formatting

Features:

• The new system DSL function allows you to run arbitrary shell commands and store them in field values. Some example usages are documented here. This is in response to issues #246 and #209.

• There is now support for ASV and USV file formats. This is in response to issue #245.

• The new format-values verb allows you to apply numerical formatting across all record values. This is in response to issue #252.

Documentation:

• The new DKVP I/O in Python sample code now works for Python 2 as well as Python 3.

• There is a new cookbook entry on doing multiple joins. This is in response to issue #235.

Bugfixes:

• The toupper, tolower, and capitalize DSL functions are now UTF-8 aware, thanks to @sheredom's marvelous https://github.com/sheredom/utf8.h. The internationalization page has also been expanded. This is in response to issue #254.

• #250 fixes a bug using in-place mode in conjunction with verbs (such as rename or sort) which take field-name lists as arguments.

• #253 fixes a bug in the label when one or more names are common between old and new.

• #251 fixes a corner-case bug when (a) input is CSV; (b) the last field ends with a comma and no newline; (c) input is from standard input and/or --no-mmap is supplied.

Note:

Thanks to @aborruso @davidselassie @joelparkerhenderson for the bug reports and feature requests!! :)