#9 allow system properties usage for cache file settings

[Jens Elkner]

it would be nice, if checkstyle.cache.file could be
used with System.properties. E.g. setting this property
to /tmp/checkstyle
would not work propperly in a multi user environment.

Setting it to a simple filename (i.e. no / in the
path), would clutter
the system an may not work properly in a multi user
environment, too.

But:
checkstyle.cache.file=/tmp/${user.name}/checkstyle.cache
OR
checkstyle.cache.file=${user.home}/tmp/checkstyle.cache

would work well, when subst the ${...} with the
appropriate System
properties...

Unfortunately I can't provide a patch, since the src
package is incomplete and thus I can't build/check the
patched one :( ...