Add color coding

[lolmaus]

E. g. work could be red, personal life could be green, web presence could be blue, etc.

[dLopreiato]

Markdown doesn't provide a way to specify color of a text, so to do this we would have to introduce some non-markdown syntax. Unless we want to specify hardcoded set of colors that one could use.

[lolmaus]

Neither does Markdown provide a syntax for dates, approximate dates and date spans. Yet somehow you managed to support them.

I suggest that a user could be able to provide a table of categories and their colors like this:

## Categories

* default: grey
* work: red
* life milestone: white
* personal life: LimeGreen
* web presence: #12f

The JS could parse this table and hide it. Or even better, turn into a legend for the convenience of readers.

The categories can be applied like this:

- life milestone: 24/02/1955 Born
- ~1968 Summer job

Or like this:

- 24/02/1955 Born
- ~1968 Summer job [work]

[lirsacc]

This could integrate well with #34 from michaelowens.

I made a quick try (see here) with his old code and a tag object in config.json (not life.md to keep parsing of the events simple).
Doesn't draw a legend for now, but could easily be added to the step.

In the interest of keeping things simple, I would also keep this limited to colors. IMO a custom stylesheet seems more appropriate for a finer control of the tags don't you think ?

I will update once #34 has been merged to account for the latest changes and make a PR.