feat: add ark update readme command

[rgee0]

Description

Adds a subcommand to update that will update the README tables in situ. This will be useful to contributors who are adding system installs / apps / tools as it will remove the need for the existing manual mechanism to update the appropriate tables in the readme. Running arkade update readme will update each of the three tables in the readme, ready to be committed with their change.

Motivation and Context

• I have raised an issue to propose this change, which has been given a label of design/approved by a maintainer (required)
  Discussed on the community call

How Has This Been Tested?

Functional

➜  arkade git:(updateReadmeTables) ✗ ./arkade update readme
Updated system table.
Updated apps table.
Updated tools table.
README tables updated successfully

README shows new format as per commit included in this change

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Documentation

• I have updated the list of tools in README.md if (required) with ./arkade get --format markdown
• [x ] I have updated the list of apps in README.md if (required) with ./arkade install --help

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have signed-off my commits with git commit -s

• I have tested this on arm, or have added code to prevent deployment