Add mechanism to programatically set general api info

[bojand]

Describe the PR

• Add dep files (Gopkg.lock and Gopkg.toml). Let me know if you want this removed please.
• The generated docs package exports SwaggerInfo variable which has Version, Description, Title, Host and BasePath. Clients can set these:

// programatically set swagger info
docs.Title = "Swagger Example API"
docs.Description = "This is a sample server Petstore server."
docs.SwaggerInfo.Version = "1.0"
docs.SwaggerInfo.Host = "petstore.swagger.io"
docs.SwaggerInfo.BasePath = "/v2"

• In the parser the default values for these are template value string ("{{.Title}}", "{{.Description}}", etc...). If they parser finds the value during parsing the defaults are replaces with the parsed value.

• Added tests for the new parsing behaviour.

• Added documentation to the README.md.

• You can find sample usage using the new swag init on an simple echo project in this repo.

Relation issue
#215

[codecov-io]

Codecov Report

Merging #220 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #220      +/-   ##
==========================================
+ Coverage   88.43%   88.49%   +0.05%     
==========================================
  Files           5        5              
  Lines        1029     1034       +5     
==========================================
+ Hits          910      915       +5     
  Misses         75       75              
  Partials       44       44

Impacted Files	Coverage Δ
parser.go	87.6% <100%> (+0.12%)	⬆️

---

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 82de8d7...2bd5d1d. Read the comment docs.

[easonlin404]

@bojand Please remove dep files (Gopkg.lock and Gopkg.toml), I think that creating another PR to support vender management it a better idea, rest LGTM. Thanks your contributions. 💯