feature_request(enhancement): install missing software

[Kristinita]

(Related issue for AppVeyor local builder)

1. Summary

It would be nice, if would be possible install software, that missed on current PC.

2. Argumentation

If:

1. User want to create build not on usual working PC
2. Or another user want to try install/build/test project with .travis.yml

User machine can't have software, that pre-installed on Travis CI. It would be nice, if would be possible simply add missing software.

3. Example of expected behavior

• wwtd.yml:

preinstall:
  apt-get:
  - nodejs
  - git

User run a command:

wwtd --preinstall

Before Travis CI steps will run file:

sudo apt-get install nodejs --yes
sudo apt-get install git --yes

If parallel would be used, it will be even better.

apt-get will install nodejs, git — names of apt-get packages.

Thanks.

[grosser]

Having a opt-in way to execute install/before_install etc steps sounds useful. Best make it generic like "wwtd --step preinstall" or so ... apt-get cannot be done in parallel and neither does travis, so we should not do it either

PR welcome, but I'm not working on this :)