Support to update a version manually in a mix.exs file

**Is your feature request related to a problem? Please describe.**

Current api allows to remove a dep or add a dep under Mix.Project.Deps. there is no option to update the version for example . Only to set extra options.

**Describe the solution you'd like**

Would be useful to have a function that can detect and update automatically a version of a package by name

**Describe alternatives you've considered**

Currently I wrote my own library to do code morphing but was hoping to move to ignitor

Start by locating the existing APIs for adding and removing dependencies under Mix.Project.Deps and how they modify mix.exs. Define the update entry point around a dependency name, then verify that its version can change while other dependency options remain intact.

Written by the indexing model from the issue text.