Fix #21 and #28

[maiertech]

This PR fixes #28 and presumably #21:

• Does not use setter to write back metadata to avoid it being cloned. This seems best practice in other plugins, e.g. metalsmith-collections.
• I cleaned up index.js and used variable names that make reading and understanding the code easier.
• Same for the tests. I added more tests. Main difference is that I do not test against generated and layout-processed files from the fixtures any more. This is not needed because I can test against files and verify anything I want to verify regarding the tag pages.
• There is a breaking change: the tags for pages has now the same structure as the global tags. This is to ensure that I can re-use a layout partial to display all tags or only page tags without having to modify how to access tag and urlSafe. This might conflict with PR Add a tagsCollection object holding a display & url safe tag label #34.
• I removed the skipMetadata option. It looked to me that it did not save much processing because the plugin still needs to create the global tags data structure.
• I updated the README and tried to explain what the plugin does under the hood. It's not perfect, but better than before.

[woodyrew]

Only use lodash.sortby rather than the whole package.

[woodyrew]

Rather than including the whole of lodash, it would be good to include the packages needed.
e.g.

var sortBy = require('lodash.sortby');

and amend line 87 to match the variable name.

[woodyrew]

Don't forget to change the dependency too:

"lodash.sortby": "^4.7.0"

[woodyrew]

@mdotasia Can you rebase to resolve the conflicts?

[jarodtaylor]

@mdotasia if you can rebase and resolve the conflicts, we'll get this PR merged in.