Implement a tagging system for notes

[gsilvapt]

Somehow find a way to create a tagging system for notes. Although I don't have a standard to recommend, something like YAML front matter could work well, as it contains enough information in there to implement things like date range search (#10), tags (this issue) and potentially other features further down the road.

This could easily scale in complexity though, as this issue is then multiple folded. At least, the following three items have to be fulfilled:

1. Implement a metadata parser to parse all note's metadata.
2. Script to automatically add metadata to each note.
3. Add to template suggestion in this repository to contain YAML front matter, which can leverage Go's templating system, so that all new notes will contain metadata.

If users decide to not use metadata, then some features may be compromised. Does it make sense to force all notes to use this metadata?