Fix #4: 403 Permission Denied

[yunginnanet]

Summary
This PR adds default HTTP headers (including a User-Agent and other relevant headers) to address the Forbidden error when scraping DoD web directories.

In essence this just reproduces the headers that my browser uses.

Changes

1. Define a constant defUserAgent to mimic a modern browser.
2. Initialize an http.Header object with several header fields:
   • User-Agent
   • Accept
   • Accept-Language
   • Upgrade-Insecure-Requests
   • Sec-Fetch-Site
   • Sec-Fetch-User
   • Accept-Encoding
   • Priority
   • Te

Potential Future Improvements
Add a list of UserAgent values and a bit of entropy to the header values in general. This should only be necessary if #4 gets reopened due to changes on DoD's end.

[Jah-yee]

Closing as duplicate of PR #6 which addresses a different issue (go install deprecation fix). Thanks for your contribution!

[Jah-yee]

Closing duplicate of PR#6 which is a cleaner, more targeted fix.

[Jah-yee]

Closing as duplicate. The go get→go install deprecation fix is better handled by PR #6 which specifically addresses the README deprecation.

[Jah-yee]

This PR (#5) fixes a 403 permission issue, but PR #6 specifically addresses the go get→go install deprecation in the README. Consider closing #5 in favor of #6 which addresses a different concern.