Added the default_member_permissions parameter to adding application commands

[sinni800]

Since changing specific application command permissions is impossible with regular bot tokens anyway, at least we can set the default required permissions to run a command on add.

[jchristgit]

@sinni800 There has been another pull request opened #30 that adds support for plenty more slash command options, including default_member_permissions as added here. Do you see a nice way to integrate the PRs with each other?

[sinni800]

I think these doesn't really need to be any integration, since the other PR supports what I do anyway