make rsshell configurable

[jgonggrijp]

I would like rsshell to be highly configurable, so the user could pick

• which underlying shell to use,
• initial working directory,
• optimization levels (e.g. PYTHONOPTIMIZE),
• the location of RED_SPIDER_ROOT (which should include all subdirectories versioned by Git),
• independently, the installer-created directories (bin, build, lib, work),

and possibly other things. The options should be accessible through commandline arguments, but should be retrieved from a configuration file when not specified. It should also be possible to specify the configuration file itself with a commandline argument so that multiple configurations can be stored in parallel. If no argument is given, rsshell should pick the configuration file that is pointed to by an environment variable (say RED_SPIDER_CONFIG) which is permanently stored in the user's environment (HKEY_CURRENT_USER on Windows, ~/.profile on unixy systems). The latter could replace the permanent storing of RED_SPIDER_ROOT. The initial configuration file should be created by setup.py, which might benefit from the parser proposed in issue #32.

Doing all of the above would allow users to, for example,

• have two local copies of the project which check out different branches, and have the same copy of rshell switch between them;
• customize the default working directory and temporarily switch to another default working directory;
• install the red spider commands in one place and install them with different build settings in another place, and allow rsshell to work with either installation.

My preferred layout for the configuration file would be JSON.

[jgonggrijp]

Removed the "low priority" from the title because of the issue referenced above.

Configuring the teleportation directory should be done first.