cli: add --include-zero flag

This flag, when used in conjunction with --count or --count-matches,
will print a result for each file searched even if there were zero
matches in that file. This is off by default but can be enabled to make
ripgrep behave more like grep.

This also clarifies some of the defaults for the
grep-printer::SummaryBuilder type.

Closes #1370, Closes #1405

complete/_rg

@@ -72,6 +72,7 @@ _rg() {
     + '(count)' # Counting options
     {-c,--count}'[only show count of matching lines for each file]'
     '--count-matches[only show count of individual matches for each file]'
+    '--include-zero[include files with zero matches in summary]'
 
     + '(encoding)' # Encoding options
     {-E+,--encoding=}'[specify text encoding of files to search]: :_rg_encodings'