Add deadline flag support in gometalinter

[gonzaloserrano]

Gometalinter has a deadline flag which defaults to 5s.

      --deadline=5s         Cancel linters if they have not completed within
                            this duration.

This PR adds a g:go_metalinter_deadline parameter to customize this time.

[fatih]

Hi @gonzaloserrano

Thanks for the addition. Can you please also add the necessary documentations? Checkout the docs/vim-go.txt file and add the necessary steps.

[gonzaloserrano]

Done, i think. Thanks.

[fatih]

Thanks @gonzaloserrano 👍