Generalise the timeout parameter in actions

[oscherler]

I have a pre-release action that takes some time (ant on a GWT project), and it sometimes triggers the Symfony Process component’s 60 s timeout. I’m thinking of making a PR to add a timeout option to the command action, but before I start working, I should ask:

• Is this desired?
• Should I add it to the composer-stability-check and composer-json-check prerequisites as well? They use the executeCommandInProcess method of BaseAction, that uses the Process component;
• Should I also set the default timeout to a higher value, to avoid having to set it on all potential long-running commands in the rmt.yml file?
• Would a global timeout option at the root level of the rmt.yml file be necessary?

Thanks for making RMT, I love it.