Add contentProvider properties for image, audio, video

[clsung]

fixes #109, #144

[codecov-commenter]

Codecov Report

Merging #292 (4515ca1) into master (01991f8) will increase coverage by 0.65%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #292      +/-   ##
==========================================
+ Coverage   77.83%   78.48%   +0.65%     
==========================================
  Files          37       37              
  Lines        2856     2868      +12     
==========================================
+ Hits         2223     2251      +28     
+ Misses        502      486      -16     
  Partials      131      131              

Impacted Files	Coverage Δ
linebot/event.go	97.88% <100.00%> (+8.93%)	⬆️
linebot/message.go	78.96% <100.00%> (+0.29%)	⬆️

---

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 01991f8...4515ca1. Read the comment docs.

[kkdai]

LGTM