Include shell completions in the executable

doc/long-help.txt

@@ -202,6 +202,9 @@ Options:
           This option exists for POSIX-compliance reasons ('u' is for 'unbuffered'). The output is
           always unbuffered - this option is simply ignored.
 
+      --completion <SHELL>
+          Show shell completion for a certain shell. [possible values: bash, fish, zsh, ps1]
+
       --diagnostic
           Show diagnostic information for bug reports.
 

doc/short-help.txt

@@ -58,6 +58,8 @@ Options:
           Only print the lines from N to M.
   -L, --list-languages
           Display all supported languages.
+      --completion <SHELL>
+          Show shell completion for a certain shell. [possible values: bash, fish, zsh, ps1]
   -h, --help
           Print help (see more with '--help')
   -V, --version