implement :q to close tab

[luanpotter]

As per suggestion on issue #260 , add the :q (or :quit) command to close the current tab.

The behavior is very similar to vim's, so I think it's a great addition.

It's good to allow for a effective option to close tab to users who would like to disable the d key (because of accidental usage).

[hoshsadiq]

Imo we should get rid of d and !d as part of this. @ueokande what do you think?

[ueokande]

@hoshsadiq
Vim-Vixen weigh a backward compatibility, heavily. Removing keys break it, and d and !d will be kept in the future releases.

[hoshsadiq]

So maybe it can be deprecated in one way or another. Anyway, no worries if it can't.

[luanpotter]

The idea is to add this option so that people can disable d in the config if so desired. IMHO there is no need to change the default behavior, as it could break expectations and also because customization is easily achieved by changing the configs.

@ueokande Is there any other consideration about the PR? I can change it if needed; would love to see it merged :)

[ueokande]

lgtm!