Added dynamic config generation for i3.

The script genconfig.bash concatenates the main configuration file with
a host specific one to create a session-specific configration file that
is then passed to i3. It therefore has to be called in .xinitrc or
something comparable before starting i3. If no host specific
configuration is found, a default file will be used instead. This is
useful for providing default that should not go into the main
configuration file, maybe because they are incompatible with some host
specific settings. For example a default bar{} block goes there, as if
this was in the main config, two i3bar instances would be started when a
host specific config specifies its own.