Add support for querying client environement variables

At connection, a remote client sends all its environement to the
server, which then provides access to client env through
kak_client_env_VAR_NAME variables in the shell.
client.kak: add a focus command for focusing a certain client.
make,grep: focus jumpclient when jumping to location
update TODO
formatting changes in command_manager.cc
Minor formatting fixes (very long lines)
return a const String& in Option::name()
client.kak: improve termcmd autodetection