Fix shortcuts types

[t0m3k]

As described in #131, using default shortcuts would cause a typescript error. This is due to the fact that [key: string]: ShortcutsItem; option was overriding today?: string; ... options.

That was also preventing intellisense from providing correct options to users.

With this PR:

• Custom options is an array of ShortcutsItem in shortcuts element named custom.
• Change to add custom options, as it is an array now it's simpler to enumerate.
• Remove question mark from fields that should not be optional.

[onesine]

Thanks for this PR @t0m3k. The problem is that the API is changing. Users will no longer be able to use the custom shortcuts in the same way. I wanted the keys for the custom shortcuts to be at the same level as the default shortcuts. But we will use this solution in the meantime.

But as I said, there will be a problem when users migrate to the new version.

[onesine]

This also poses another problem. A user cannot put a custom shortcut before a default shortcut.

[t0m3k]

Thanks for comments @onesine, I have fixed both points in PR #138

And sorry for uploading this without giving it enough thought,

[onesine]

Don't worry @t0m3k. Thanks for the new PR