♻️ make core independent from the git client.

[bpetetot]

In order to use gitmoji-changlelog with other git clients (like a http client or other), we need to extract the dependency with the current git client (reading .git on the filesystem).

Here is a naive refactoring extracting all calls done by the current git client.

First step for #168

You can now call the core with:

generateChangelog(from, to, { client: myGitClient }) 

The client myGitClient must have 2 functions:

getTags() : Promise([string])
getCommits(fromTag: string, toTag: string) : Promise([{ hash, author, date, subject, body }])