Support merging threads (`hey merge`)

[arnthorsnaer]

HEY's Merge Threads feature isn't currently exposed in the CLI.

Requested

A command to merge two or more threads, e.g.:

hey merge <posting_id_1> <posting_id_2> [--title "New thread title"]

Notes

• Since merges are irreversible, a --confirm flag or interactive prompt would be a nice safety net
• Similar to #74 — may also require API support from HEY