Add publish stage in CI

[JimiC]

This PR automates the publication process.

Steps:

• The member constructs the CHANGELOG.
• The member performs locally npm version <version> --no-git-tag-version.
• The member submits the PR for approval.
• Upon approval from another member, the member merges the PR.
• The member pulls the latest commit locally.
• The member tags the merged commit.
• The member pushes the tag.
• CI picks up the tag and does the publication.

Note:

• member is a member of the @vscode-icons/development team.

Requirements:
@robertohuertasm In order for this to work a secure environment variable MUST be set in travis settings, named VSCE_TOKEN with a value of the vsce publisher token. It's recommended to generate a fresh token, so it lasts longer, as they have a max year long lifetime.

Additionally, the extensionSettings version has been automated. It will always point to the manifest (package.json) version.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@0e7d6a1). Click here to learn what that means.
The diff coverage is 100%.

@@           Coverage Diff            @@
##             master   #1627   +/-   ##
========================================
  Coverage          ?    100%           
========================================
  Files             ?      50           
  Lines             ?    2849           
  Branches          ?     138           
========================================
  Hits              ?    2849           
  Misses            ?       0           
  Partials          ?       0

Impacted Files	Coverage Δ
src/settings/extensionSettings.ts	100% <100%> (ø)

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[codeclimate]

Code Climate has analyzed commit 12fe551 and detected 0 issues on this pull request.

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[robertohuertasm]

Added already the VSCE_TOKEN to Travis