[Taxonomy] Allow plug-ins to introduce bootstrap components

This pull request allows plugins to introduce their own bootstrap components (top-level components that show up inthe user environment.) Previously, this was not feasible for reasons identified in nasa/mct#207

Changes are:
* Add "getBootstrapComponents" to ComponentProvider API. Plugins may use this method to expose top-level components to the system.
* Add Bootstrap capability. Component types may expose this capability to control how and in what order they appear as bootstrap components. (Top-level components which do not implement this capability are still handled, albeit with a lesser degree of support for handling their sorting.)

 Wrote or re-wrote unit tests (Y/N): Y
 Ran entire suite of unit tests successfully (Y/N): Y
 Verified bug fix with the described replication steps or verified the implementation meets the requirement cited (Y/N): Y
 Code coverage requirements satisfied (new code=80%, changed code=at least as high as existing) (Y/N): Y
 Code review date and attendees: Self-review (team size = 1)
 Describe potential impact on other areas of the code: Component bootstrapping
 Describe any test prerequisites for this fix (e.g., restart the database): Run with plugin that injects its own bootstrap components
 For bugs marked "Regression - Yes", describe the cause of the regression: N/A
 Did you modify any pom.xml file? If yes, did you thoroughly document your changes and reasons? N