[Feature] Stats from last execution

[grantcarthew]

Hi @mrvautin.

I would like to know the change since I last executed authorStats.

I know of sites like npm-stats however this would be handy on this little package of yours.

I am not talking about a long history of statistics. Just something like this:

Package name	Daily Downloads	Weekly Downloads	Monthly Downloads	Dependants
elephant	194 (182)	1549 (1443)	7468 (6439)	30 (24)

So this would involve saving the data object into a local temp cache file and reading this file on execution. Updating it if the data is from a different day.

It could be done differently such as a change number rather than the last number 2345 (up 345) with emoji and the like.

What do you think?