config: add clone options

[camandel]

A try to fix issue #3 adding a new optional parameter ("options") in clone "step".
Example:

        steps:
          - clone:
              options: "--recurse-submodules --depth 1"

[sgotti]

@camandel Thanks for the PR. There are two different ways to implement what is asked in #3

• This way: an option to specify git clone arguments.
• Just set some options to the clone step that will be converted to git arguments (i.e depth) and let the user do everything else in a successive run step (i.e. git submodule update --recursive --remote)

This former is more powerful but also more confusing since the user can pass everything as "options".

The latter is less error prone and won't clash with possible changes to the clone scripts generated by agola. For example, if in future we want to clone with a default user defined global/per project depth we could do this, with the former it'll be difficult since it'll depend on the arguments order/override logic if the user already provided a depth as an option.

User can always use a custom clone step if they want to do special things.

[camandel]

@sgotti code updated to use the second approach. At the moment only these two parameters can be converted to git arguments:

        steps:
          - clone:
              depth: 1
              recursesubmodules: true

[sgotti]

@camandel LGTM. Merging. Thanks!