add description (show, add, modify)

[psethwick]

adds --description option to add and modify commands description listed in show command

let me know if there are any issues you'd like me to address, I'm pretty fresh on go

addresses #185

[HacDan]

Seems all functional.

My only issue is no short command like -d. I know -d is taken by -date but it would be nice to have a shorter way of adding a description. These are described in Aliases here: https://github.com/sachaos/todoist/blob/2f80bdc65de44581c4497107a092c73f39ae0b62/main.go#L61

I, unfortunately, do not have any suggestions, but that would be the only fault I see. Rest looks good to me! I don't think not having an alias is enough to hold this PR back, though!

[psethwick]

I did have -d in there to start with, but the conflict with -date stopped it compiling (actually I don't remember, might have been runtime). Suggestions, in no real order:

• de
• D
• e

[HacDan]

I think -D could work, but I'm open to other options as well. I mainly use this from a scripting standpoint, so not a huge effect on my personal use, I only brought it up for consistency with the other flags.

EDIT: Clearly missed the shift key there.

@kenliu would be the one making the call here I believe.