Support HTTP/2 h2c mode (cleartext)

[ochanism]

I've added a function "StartH2cServer" for HTTP/2 with h2c mode.
I'd like to run a HTTP/2 echo server on a secure private network, so I don't need a TLS.
However, echo requires a TLS to run a HTTP/2 server, so I've added the function using a h2c library.
Please review my small changes.

[codecov]

Codecov Report

Merging #1489 into master will increase coverage by 0.03%.
The diff coverage is 88.23%.

@@            Coverage Diff             @@
##           master    #1489      +/-   ##
==========================================
+ Coverage   84.35%   84.38%   +0.03%     
==========================================
  Files          27       27              
  Lines        2077     2094      +17     
==========================================
+ Hits         1752     1767      +15     
- Misses        212      213       +1     
- Partials      113      114       +1

Impacted Files	Coverage Δ
echo.go	85.49% <88.23%> (+0.15%)	⬆️

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[vishr]

• Can you call it StartH2CServer ?
• Do you mind updating the docs on echox project

[ochanism]

• Can you call it StartH2CServer ?

Ok

• Do you mind updating the docs on echox project

Ok I'm going to update it soon.

[ochanism]

I've changed the function name and I've made a pull request for a document.
Please refer to labstack/echox#132