Adding new checkup package to provide plugin utils. Generates docs

[scalvert]

Implements #734

Adds a new package, @checkup/plugin, to provide plugin utilities for plugins to use. The first util is a new command, which generates markdown documentation files for tasks.

The first iteration generates very rudimentary documentation, but paves the way for additions to improve the quality of the plugin docs.

Example:

Generates...

<!--TASK_NAME_START-->
# javascript/outdated-dependencies
<!--TASK_NAME_END-->

<!--TASK_DESCRIPTION_START-->
Gets a summary of all outdated dependencies in a project
<!--TASK_DESCRIPTION_END-->

<!--RUN_START-->
## To run this task

\`\`\`bash
checkup run --task javascript/outdated-dependencies
\`\`\`
<!--RUN_END-->

which looks like...

javascript/outdated-dependencies

Gets a summary of all outdated dependencies in a project

To run this task

checkup run --task javascript/outdated-dependencies