Add Support for Conversations API

[hiromi-mi]

Support conversations API included from Mastodon 2.6.0 (tootsuite/mastodon#8832, https://docs.joinmastodon.org/methods/timelines/conversations/).

• GET /api/v1/conversations GetConversations()
• DELETE /api/v1/conversations/:id DeleteConversation()
• POST /api/v1/conversations/:id/read MarkConversationAsRead()

[codecov-io]

Codecov Report

Merging #126 into master will increase coverage by 0.01%. The diff coverage is 81.81%.

@@            Coverage Diff            @@
##           master    #126      +/-   ##
=========================================
+ Coverage   80.09%   80.1%   +0.01%     
=========================================
  Files          29      29              
  Lines        1492    1503      +11     
=========================================
+ Hits         1195    1204       +9     
- Misses        204     205       +1     
- Partials       93      94       +1

Impacted Files	Coverage Δ
status.go	85.62% <81.81%> (-0.29%)	:arrow_down:

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 1516135...6a58c77. Read the comment docs.

[mattn]

LGTM, Thank you.