Rename BashCompDirectives to ShellCompDirectives (#1082)

Since the completion directives will be used for all shells, and that
these names will be consumed by users, this is a more appropriate name.

Signed-off-by: Marc Khouzam <marc.khouzam@montreal.ca>

bash_completions.go

@@ -344,7 +344,7 @@ __%[1]s_handle_word()
     __%[1]s_handle_word
 }
 
-`, name, CompRequestCmd, BashCompDirectiveError, BashCompDirectiveNoSpace, BashCompDirectiveNoFileComp))
+`, name, ShellCompRequestCmd, ShellCompDirectiveError, ShellCompDirectiveNoSpace, ShellCompDirectiveNoFileComp))
 }
 
 func writePostscript(buf *bytes.Buffer, name string) {