Updates for Bot API 5.0 and 5.1

[Syfaro]

Closes #390. Would love to get feedback on if I've covered all of the changes correctly!

[codecov]

Codecov Report

Merging #392 (fcf56ff) into develop (366879b) will decrease coverage by 0.42%.
The diff coverage is 44.89%.

@@             Coverage Diff             @@
##           develop     #392      +/-   ##
===========================================
- Coverage    49.88%   49.45%   -0.43%     
===========================================
  Files            7        7              
  Lines         1287     1379      +92     
===========================================
+ Hits           642      682      +40     
- Misses         609      653      +44     
- Partials        36       44       +8     

Impacted Files	Coverage Δ
types.go	77.35% <ø> (ø)
configs.go	39.77% <42.16%> (-0.10%)	⬇️
bot.go	43.09% <55.55%> (+0.38%)	⬆️
helpers.go	60.40% <66.66%> (+0.36%)	⬆️

---

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 366879b...3b5c8a9. Read the comment docs.

[tjhorner]

AFAICT, it is currently impossible to receive the chat_member update via getUpdates, since it's required to pass that into the allowed_updates parameter to receive them. But from what I can see, it's not currently possible to set that parameter:

telegram-bot-api/configs.go

Lines 889 to 894 in 24d4f79

	// UpdateConfig contains information about a GetUpdates request.
	type UpdateConfig struct {
	Offset int
	Limit int
	Timeout int
	}

We should add this parameter in, then add constants for each of the valid update types.