New tags handling, issue #22

[libremente]

Title

This PR changes the whole tags handling tackling issue #28.

Content

• 2 new files are introduced: categories-list.rst and scope-list.rst.
• freeTags are removed to allow for better indexing etc.
• the tags key has been replaced with the category one where each entry must be a valid value in categories-list
• the standaloneType key is introduced in order to help defining the type of software
• the intendedAudience/scope key is introduced as an optional key where each entry must be a valid value in scope-list

This is a breaking change.

Review

• Ensure your files are written following RST specs (not MD!)
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