Variables, begin/end blocks, pattern-action blocks

This major release dramatically expands the expressive power of Miller's put DSL. The TL;DR is that you can now write things like

mlr put '@x_sum += $x; end { emit @x_sum }'

For full details please see the following reference sections:

• http://johnkerl.org/miller/doc/reference.html#put
• http://johnkerl.org/miller/doc/reference.html#Out-of-stream_variables_for_put
• http://johnkerl.org/miller/doc/reference.html#Pattern-action_blocks_for_put
• http://johnkerl.org/miller/doc/reference.html#Begin/end_blocks_for_put
• http://johnkerl.org/miller/doc/reference.html#Indexed_out-of-stream_variables_for_put
• http://johnkerl.org/miller/doc/reference.html#Emit_statements_for_put
• http://johnkerl.org/miller/doc/reference.html#Unset_statements_for_put

as well as the following cookbook section:

• http://johnkerl.org/miller/doc/cookbook.html#Using_out-of-stream_variables

Additional minor features in Miller 4.0.0:

• Compound assignment operators such as +=, <<=, etc. are not new but were not previously announced in a release note.
• Double-backslashing behavior for sub and gsub has been fixed: echo 'x=a\tb' | mlr put '$x=sub($x,"\\t","TAB")' now prints aTABb as desired. (The underlying issue was an unfortunate interaction between Miller's backslash-handling and the system regex library's backslash-handling.)
• As an alternative to specifying input files as the last items on the Miller command line, you can now specify a single input file before other command-line switches and verbs using --from: for example, mlr --from myfile.dat put '$z = $x + $y' then stats1 -a sum -f z. The context is simple keystroke-reduction for interactively appending then-chains by up-arrowing at the command line: it's easier to iterate when you don't have to left-arrow past the input file name.