Add datetime string to span element as title

[mschoettle]

This is a simple solution to get the datetime on hover over the <span> element.

Alternative/additional solution

GitHub and GitLab include the timezone as well:

GitHub

GitLab

We could also consider having the title include the timezone (although I realized that depends on the presence of the timezone setting as well, otherwise it would be UTC) [side question here: would it be better to default to the system timezone here?]

Fixes #131

[timvink]

Thanks for the contribution !

[timvink]

It seems we need to update some unit tests also

[timvink]

And tz is tricky as browsers might convert it to local timezone. Good to add.

[mschoettle]

Sorry, I made a silly mistake. Now the tests run through.

And tz is tricky as browsers might convert it to local timezone. Good to add.

Good to add the timezone to the string? Would you prefer this as a separate date format or part of datetime?

[mschoettle]

Ping @timvink

Is this good as is or should we add the timezone as well (see questions above)?

[timvink]

It's good as-is, but would be better to have the timezone information on hover also. Separate PR?