added twanager option to activate stack traces

that's usually easier than setting `twanager.tracebacks` in the
respective configuration file

opted for `--tb`, inspired by py.test

caveats:
* like `--load`, this remains undocumented (due to the lack of
  facilities for such documentation?)
* reinvents options parsing, in a fairly primitive manner
* required refactoring `_external_load` for improved encapsulation
refactored options parsing
Merge pull request #94 from tiddlyweb/twanager-options

added --tb twanager option for stack traces