Support map as query string or post form parameters

[thinkerou]

ref: #1362 #1106

please @javierprovecho @appleboy review the idea, thanks!

when use the following curl command:

curl -X POST 'http://localhost:8888/post?ids\[a\]=1234&ids\[b\]=hello&ids\[c\]=3.14'

and use c.QueryMap("ids") will return the following result:

ids: map[c:3.14 a:1234 b:hello]

• Add query map for post form
• Add unit test
• Add document
• Support post form map for go1.6

[codecov]

Codecov Report

Merging #1383 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1383      +/-   ##
==========================================
+ Coverage   98.01%   98.03%   +0.02%     
==========================================
  Files          36       36              
  Lines        1861     1886      +25     
==========================================
+ Hits         1824     1849      +25     
  Misses         30       30              
  Partials        7        7

Impacted Files	Coverage Δ
context.go	96.38% <100%> (+0.24%)	⬆️

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