Custom header support during reload: closes #60

[cboitel]

Main changes made:

• loading is now performed in a common func to watch/__reload
• align messages on console when reloading from watch or __reload

Tests OK

2020/02/06 10:41:17 GET /test (Test) => 200 (application/json)
2020/02/06 10:41:17 GET /test (Test) => 200 (application/vnd.test-api+json)
2020/02/06 10:41:17 GET /test (Test) => 200 (application/yaml)
2020/02/06 10:41:17 GET /test (Test) => 200 (application/x-yaml)
2020/02/06 10:41:17 GET /test (Test) => 200 (application/vnd.test-api+yaml)
2020/02/06 10:41:17 GET /test (Test) => 200 (text/yaml)
2020/02/06 10:41:17 GET /test (Test) => 200 (text/x-yaml)
2020/02/06 10:41:17 GET /test (Test) => 200 (text/vnd.test-api+yaml)
2020/02/06 10:41:17 GET /test (Test) => 200 (text/vnd.test-api+xml)
2020/02/06 10:41:17 Cannot marshal as 'text/vnd.test-api+xml'!
2020/02/06 10:41:17 GET /test (Test) => 200 (application/json-with-extensions)
2020/02/06 10:41:17 Cannot marshal as 'application/json-with-extensions'!
PASS
ok      github.com/danielgtaylor/apisprout      0.414s

Manual tests (on Windows):

• use an invalid local or http swagger file
  • displays error and exists
• use a local swagger file and enable watch on it
  • change file => reloading occurs and succeeds
  • rename file => reloading occurs and fails but original API keeps on being served
  • rename back file => reloading occurs and succeeds
  • change file again after renames => reloading occurs and succeeds
• use a HTTP swagger file with custom header
  • initial load ok
  • call __reload: reload occurs and succeeds
  • file moved on server and __reload called: reloading occurs and fails but original API keeps on being served
  • file moved back on server and __reload called: reloading occurs and succeeds