go run grep.go [-flags] [file.txt]
-A - "after" - print +N strings after match
-B - "before" - print +N strings before match
-C - "context" - print ±N (A+B) strings around match
-c - "count" - print count of matched strings
-i - "ignore-case" - ignore case
-v - "invert" - instead of match, exclude
-F - "fixed" - exact string full-match
-n - "line num" - print number of matched string
