Add content script to build file

[Snuupy]

Status:
Done and working
waiting for travis CI build
waiting on a local build of brave-browser to complete for testing.

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
  • Add a block this element feature to the brave-extension (cosmetic adblock filters) brave-browser#94
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.
• Request a security/privacy review as needed.
• Add appropriate QA labels (QA/Needed or QA/No-QA-Needed) to include the closed issue in milestone

Test Plan:

1. npm run build && npm run start
2. right click context menu to make sure you can block elements through CSS selectors

Reviewer Checklist:

• New files have MPL-2.0 license header.
• Request a security/privacy review as needed.
• Adequate test coverage exists to prevent regressions
• Verify test plan is specified in PR before merging to source