Feature markdown description

[Nerzal]

Describe the PR
Added new annotation "@description.markdown" which parses the general api description from a api.md file in the given directory
Added new annotation "@tag.description.markdown" which parses the tag description from a tagname.md file in the given directory
Added a new parameter for the cli which takes the markdown directory path
Added 2 test methods for the stuff above
Fixed some linter warnings.

Additional context
This addition is mainly for the use with redoc, as redoc is able to render markdown, which allows us to create stunning documentations using code annotations in combinition with markdown files stored in the project. <3

Link to redoc: https://github.com/Rebilly/ReDoc

[Nerzal]

@easonlin404 beside the broken CI Build.
Do you find that feature useful?
If so, i can fix the issues with CI etc.

[codecov-io]

Codecov Report

Merging #377 into master will decrease coverage by 1.05%.
The diff coverage is 55.31%.

@@            Coverage Diff             @@
##           master     #377      +/-   ##
==========================================
- Coverage   84.71%   83.65%   -1.06%     
==========================================
  Files           7        7              
  Lines        1341     1383      +42     
==========================================
+ Hits         1136     1157      +21     
- Misses        119      130      +11     
- Partials       86       96      +10

Impacted Files	Coverage Δ
gen/gen.go	60.78% <40%> (-8.11%)	⬇️
parser.go	82.33% <59.45%> (-1.11%)	⬇️

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[Nerzal]

Here is an example, of a markdown tag description i created using the features in this pr

[easonlin404]

@Nerzal Thanks for your contributions. 👍 IMO, it's a useful feature to support markdown descriptions.
Would you please add example/at readme to demonstrate how to use? And this PR we needs more time to review it since having change a lot.

[easonlin404]

leave comment, thx.

[pei0804]

I secure time to see.

[Nerzal]

I'll try to add to add examples + cleanup the branch in the next days. Currently only have little time caused by workload :D

[Nerzal]

I merged master branch again, cause of merge conflicts.

[josephzidell]

@Nerzal Just stumbled across this beautiful PR. How can you use redoc with swaggo?

[Nerzal]

Heyho, i'll add another pr soon, which introduces this feature.

In the meantime you can jut add this and add deliver the HTML content of the constant as response.

package docs

// RedocDocumentation is a static HTML used to deliver the redoc API-Documentation
const RedocDocumentation = `

<title>Your Title</title>

<!--
ReDoc doesn't change outer page styles
-->
<style>
  body {
    margin: 0;
    padding: 0;
  }
</style>

<script src="https://cdn.jsdelivr.net/npm/redoc@next/bundles/redoc.standalone.js"> </script> `

See #396 as reference issue.