add mint-repl and associated readme, docs

[dan-da]

A note about naming:

I was just going to name this example "mint".

However, In order to keep the code simpler and contained in a single file, I've decided to implement cli arguments feature as a separate example. As such, we will end up with mint-cli example and mint-repl example. The mint-cli example will require persistence to disk, which the repl does not.

[dan-da]

@davidrusu I made the suggested changes. In particular, amount validation is gone, although it does warn if amount is 0 or if sum(outputs) is greater than sum(inputs). I verified that sn_dbc give an error for this case, and also that it issue 0 amount certs.