Add stream support for bp.add_route()

[chenjr0719]

This PR adds stream argument for bp.add_route() to provide another solution if someone wants to using blueprint with request stream.

[chenjr0719]

Just a record, Travis is failed at the lint check now. I think the reason is new releases of flake8(3.7.0 and 3.7.1). Try to fix it now.

[codecov]

Codecov Report

Merging #1482 into master will decrease coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1482      +/-   ##
==========================================
- Coverage   91.44%   91.38%   -0.06%     
==========================================
  Files          17       17              
  Lines        1730     1730              
  Branches      328      328              
==========================================
- Hits         1582     1581       -1     
  Misses        123      123              
- Partials       25       26       +1

Impacted Files	Coverage Δ
sanic/response.py	100% <100%> (ø)	⬆️
sanic/router.py	95.43% <0%> (-0.46%)	⬇️

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[sjsadowski]

Looks good to me