Support twitter new char limit

[chocoelho]

Twitter has increased its character limit to 280 chars per tweet. This PR adds support for it and resolves #15.

[codecov-io]

Codecov Report

Merging #16 into master will increase coverage by 0.07%. The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #16      +/-   ##
=========================================
+ Coverage   94.23%   94.3%   +0.07%     
=========================================
  Files          10      10              
  Lines         243     246       +3     
  Branches       14      14              
=========================================
+ Hits          229     232       +3     
  Misses         13      13              
  Partials        1       1

Impacted Files	Coverage Δ
knowledge_share/twitter_helpers.py	100% <100%> (ø)	:arrow_up:
knowledge_share/models.py	94.33% <100%> (+0.33%)	:arrow_up:

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[filipeximenes]

Hey, how about making the char limit a settings variable? This would make the package more flexible. Also, there should be information about this in the README

[chocoelho]

I'm with you on this one @filipeximenes.

[chocoelho]

In this case, shouldn't both TWITTER_MAX_CHARACTER and TWITTER_URL_SHORTENER_SIZE be env variables @filipeximenes?

[chocoelho]

Or better yet, make use of @lucabezerra PR?

[filipeximenes]

Yes, let's move both to the settings. Let's keep the automatic integration in a separate PR

[fjsj]

I'll merge this anyway and create an issue for those improvements.