Add 'insert' command to insert arbitrary features in a simplified way

This PR adds a new command that makes it easier (and generally more robust than using import commands or applying a patch) to insert a feature or a set of features in an existing tree.

This is specially targeted at GeoGit clients wanting to update the repo with the changes made to an exported layer, without having to use any of the supported import formats or deal with the complexity of creating a patch.

Thi is not supposed to replace any import operation, and it works only under certain assumptions (the tree already exists, the inserted feature uses the default schea of the tree...). The idea behind this PR is to add a tool that simplifies the most common use case of editing a given tree.