Allow to use custom extra_plugins

[julien-bouquet]

By example, use:

          extra_plugins: |
          @semantic-release/commit-analyzer
          @semantic-release/git
          @semantic-release/github

Instead of

extra_plugins: |
          @semantic-release/commit-analyzer
          @semantic-release/release-notes-generator
          @semantic-release/git
          @semantic-release/github

[rlespinasse]

For that, I plan to wait until #3 implementation in order to use the configuration to calculate the needed extra_plugins based on repository configuration to stay opinionated 😄

With this issue, the default (and opinionated) configuration will be

extra_plugins: |
          @semantic-release/commit-analyzer
          @semantic-release/release-notes-generator
          @semantic-release/git
          @semantic-release/github

And if you have (let say) a .releaserc file with the following content

{
  "plugins": [
    "@semantic-release/commit-analyzer",
    "@semantic-release/release-notes-generator",
    "@semantic-release/npm",
    "@semantic-release/git"
  ]
}

Then the action extra_plugins configuration will be like

extra_plugins: |
          @semantic-release/commit-analyzer
          @semantic-release/release-notes-generator
          @semantic-release/npm
          @semantic-release/git

In addition, the following plugins are already bundled, we do not need to add it as extra_plugins in the associated sub-action

"@semantic-release/commit-analyzer"
"@semantic-release/release-notes-generator"
"@semantic-release/npm"
"@semantic-release/github"