add --strategy to choose the strategy to use

`--strategy random` or `--strategy binpacking:<overcommit>`

default: `--strategy binpacking:0.05`
Minor nitpick:

This **might** be more clean if written as:

```go
map[string]PlacementStrategy {
    "binpacking": &BinPackingPlacementStrategy{},
}
```

However, I might be wrong, it's a matter of personal preference.