Make ECDSA the default for issuer and end-entity keys

[aarongable]

Make ECDSA the default algorithm for the "-ca-alg" flag, replacing RSA. Also plumb that algorithm through to the end-entity key generation, making that configurable (and also defaulting it to ECDSA). This significantly speeds up minica by default.

$ git checkout origin/master
$ go build .
$ time (for i in {1..100}; do ./minica -domains $i.example.com; done)               
22.94s user 1.04s system 100% cpu 23.776 total

$ git checkout ecdsa-default
$ go build .
$ time (for i in {1..100}; do ./minica -domains $i.example.com; done)
0.76s user 0.56s system 109% cpu 1.204 total

Fixes #29