Filter qbt list command by torrent state.

[xel86]

Allows for the qbt list command to be filtered to list only the torrents specified by a state argument, like downloading.

For example:
qbt list --filter=active will list all active torrents currently downloading or uploading.
qbt list will continue to list all torrents.

New flags:

• --filter,-f - takes a single qbit state as filter
• --category,-c - takes a single category
• --tag,-t - takes a single tag
• --hashes,-h - takes one or multiple hashes separated by | pipe: "hash1|hash2"

Also removed the uploading TorrentFilter from domains.go since it is not a valid filter for the qbittorrent api, and added the missing errored filter. (https://github.com/qbittorrent/qBittorrent/wiki/WebUI-API-(qBittorrent-4.1)#get-torrent-list)

Fixes #46

[ludviglundgren]

Hello! Thanks for the PR :)

Left some feedback on good and simple improvements!

[ludviglundgren]

I did just a bit of refactoring 😄

Changed the args from state to filter and also added category, tag, hashes.

Updated the qbit client to take a context everywhere, so requests can be cancelled.

More to come!

[xel86]

Sorry about that, been very busy lately and completely forgot about this PR. Thanks for the improvements :)