Introduce git_recursive option

[rskuipers]

Q	A
Bug fix?	No
New feature?	Yes
BC breaks?	No
Deprecations?	No
Fixed tickets	N/A

This PR introduces an option to enable or disable the --recursive flag while cloning. The use case is that we only have submodules for development related stuff, so dropping the --recursive flag would be more performant and it would keep us from having to include deploy keys for those git repositories as well.

set('git_recursive', true);

➤ Executing task deploy:update_code
...
[example.com] > /usr/bin/git clone  --depth 1 --recursive -q git@github.com:deployphp/deployer.git /srv/http/foobar/releases/9 2>&1

set('git_recursive', false);

➤ Executing task deploy:update_code
...
[example.com] > /usr/bin/git clone  --depth 1  -q git@github.com:deployphp/deployer.git /srv/http/foobar/releases/8 2>&1

I didn't see any tests I could extend with this new option, let me know if I missed something.

I've also created a PR to update the documentation: deployphp/docs#102

[antonmedv]

Nice!