use exceptions instead of prints
This has the big benefit that we can use those methods even when we do not want to print something to stdout or stderr like in web interface.
fix config parser
add command to get and set a config value

Examples:

$ git tessera config user.email # get value of setting user.email
$ git tessera config user.email "tuxtimo@gmail.com" # set value of setting user.email
Merge branch 'master' of https://github.com/neolynx/git-tessera