[Enhancement] Customize rebuild gtags database cmd

[ywang-wnlo]

When I used this extension for reading Linux kernel, the autoUpdate would be failed.

$ global -u
Warning: cannot stat 'vmlinux-gdb.py'. ignored.
gtags: cannot read file 'signing_key.pem'.

Because the signing_key.pem file cannot read. Would you like to enhance the extension by providing customize rebuild gtags database cmd, or just change the global -u to gtags -i --skip-unreadable