use a default port, and allow seeds to be passed as a string

before, passing seeds in on command line looked like this:

``` js
./client.js server --seeds.host grimwire.com --seeds.port 2000
```
but this pull request simplifies to
``` js
./client.js server --seeds grimwire.com:2000
```
Also, you can leave off the default port

``` js
./client.js server --seeds grimwire.com
```
if you have more than one seed server, it is like this:
``` js
./client.js server --seeds grimwire.com --seeds gromwera.com
```
(minimist turns that into an array `seeds: [...]`