#286 More flexible treasure generation

[DraugTheWhopper]

A few things that would be useful to have during artifact generation:

• Allow specifying an artifact as a modifier
  For example:

arch potion_generic
artifact minor healing

• Allow float values for "chance"
  For example:

list uncommon_artifacts
chance 0.1

• Allow setting a new magic level (or an offset) when calling a new treasurelist
  For example:

list special_uncommon
set_magic 5

or:

list special_uncommon
set_magic +10

[Nicolas Weeger]

Magic adjustment options added (list_magic_value and list_magic_adjustment) for lists in commit 2ddf00414d6233f004936fed142f2f7a0670d969

Artifact generation added (artifact) in commit 8e2da59b494086c6ade6dc191541be7a7d1b50b4