Request - add start-on-deploy flag to [deploy] section

[reubenfirmin]

Describe the bug
If all machines for an app are down, doing a fly deploy will not start new machines. This can be desireable behavior, but it could also be useful to always start machines after a deploy.

** Suggestion**
In the [deploy] section of the toml, add a new flag along the lines of start-on-deploy, which, if true, will always start (at least) the minimum number of machines configured for the app.