Add ReferrerPolicy to Secure middleware

[htdvisser]

This Pull Request adds Referrer-Policy to the Secure middleware.

The goal of this header is to hide potentially sensitive referrer information (such as the origin or path of the original request).

Example:

If a user visits the hypothetical https://echo.labstack.com/?some_secure_token=foobarbaz, a request is made to https://use.fontawesome.com/releases/v5.0.8/js/all.js. This request contains a header Referer: https://echo.labstack.com/?some_secure_token=foobarbaz. FontAwesome does not need to know some_secure_token to give us icons, so we should limit the information they get from us in the Referer header. This can be done by setting a Referrer-Policy header.

[codecov]

Codecov Report

Merging #1363 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1363      +/-   ##
==========================================
+ Coverage   84.15%   84.17%   +0.01%     
==========================================
  Files          27       27              
  Lines        2058     2060       +2     
==========================================
+ Hits         1732     1734       +2     
  Misses        212      212              
  Partials      114      114

Impacted Files	Coverage Δ
echo.go	85.62% <ø> (ø)	⬆️
middleware/secure.go	93.75% <100%> (+0.41%)	⬆️

---

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 01229ec...028132d. Read the comment docs.