Preserve edits on .cargo/config.toml

[andrefigas]

Fix #26

This PR will allow the end user to keep their changes on the .cargo/config.toml if those changes de placed out of the auto-generated block

.cargo/config.tml

#They can place their changes here: before the auto-generated block

#<RustJNI>
#auto-generated code


#</RustJNI>

#And here: after that