add -F filter by column flag (closes #13)

tablizer.1

@@ -133,7 +133,7 @@
 .\" ========================================================================
 .\"
 .IX Title "TABLIZER 1"
-.TH TABLIZER 1 "2023-11-22" "1" "User Commands"
+.TH TABLIZER 1 "2024-05-07" "1" "User Commands"
 .\" For nroff, turn off justification.  Always turn off hyphenation; it makes
 .\" way too many mistakes in technical documents.
 .if n .ad l
@@ -155,6 +155,7 @@ tablizer \- Manipulate tabular output of other programs
 \&      \-s, \-\-separator string   Custom field separator
 \&      \-k, \-\-sort\-by int        Sort by column (default: 1)
 \&      \-z, \-\-fuzzy              Use fuzzy seach [experimental]
+\&      \-F, \-\-filter field=reg   Filter given field with regex, can be used multiple times
 \&
 \&    Output Flags (mutually exclusive):
 \&      \-X, \-\-extended           Enable extended output
@@ -264,8 +265,8 @@ Sorts timestamps.
 .PP
 Finally the  \fB\-d\fR option  enables debugging  output which  is mostly
 useful for the developer.
-.SS "\s-1PATTERNS\s0"
-.IX Subsection "PATTERNS"
+.SS "\s-1PATTERNS AND FILTERING\s0"
+.IX Subsection "PATTERNS AND FILTERING"
 You can reduce the rows being  displayed by using a regular expression
 pattern.  The  regexp is  \s-1PCRE\s0 compatible, refer  to the  syntax cheat
 sheet here: <https://github.com/google/re2/wiki/Syntax>.  If you want
@@ -300,6 +301,21 @@ Example for a case insensitive search:
 You  can use  the experimental  fuzzy seach  feature by  providing the
 option \fB\-z\fR, in which case the  pattern is regarded as a fuzzy search
 term, not a regexp.
+.PP
+Sometimes you want to  filter by one or more columns.  You can do that
+using the \fB\-F\fR option. The option can be specified multiple times and
+has the following format:
+.PP
+.Vb 1
+\&    fieldname=regexp
+.Ve
+.PP
+Fieldnames (== columns headers) are case insensitive.
+.PP
+If you specify more than one filter, both filters have to match (\s-1AND\s0
+operation).
+.PP
+If the option \fB\-v\fR is specified, the filtering is inverted.
 .SS "\s-1COLUMNS\s0"
 .IX Subsection "COLUMNS"
 The  parameter  \fB\-c\fR  can  be  used  to  specify,  which  columns  to
@@ -487,7 +503,7 @@ or to submit a patch, please open an issue on github:
 .IX Header "LICENSE"
 This software is licensed under the \s-1GNU GENERAL PUBLIC LICENSE\s0 version 3.
 .PP
-Copyright (c) 2023 by Thomas von Dein
+Copyright (c) 2022\-2024 by Thomas von Dein
 .PP
 This software uses the following \s-1GO\s0 modules:
 .IP "repr (https://github.com/alecthomas/repr)" 4