cli: add support for actions in popper search

[ivotron]

Bring back the functionality of the popper search command so that it is able to index and search actions contained in a pre-defined list of repositories (e.g. all repositories in the popperized organization). This list of repositories containing actions is written in the .popper.yml file and can be modified by the user

[JayjeetAtGithub]

@ivotron Will the popper search functionality in popper 2.0 be the same as that of popper 1.x ?

[ivotron]

Yes, the idea is to allow users to do popper search <keyword> so that they can search for actions by matching the given keyword to the name of an action. An --include-readme flag then instructs popper to search also by looking at the content of the README file.

A list of action repos is defined in a search_sources attribute of the .popper.yml file. For example:

search_sources:
- github.com/popperized/zenodo
- github.com/popperized/cloudlab
- github.com/actions/aws
- github.com/actions/gcloud
..

We can have a list of repos specified by default. An alternative would be to have that list be defined in a repo, and retrieve it everytime we do a popper search --update-sources. This latter can be left out of the scope of this issue (we might want to create a separate issue for that).