Add extension points to preprocess2

[jelovirt]

Description

Add Ant target extension points to preprocess2:

• preprocess2.topics.pre
• preprocess2.topics.post
• preprocess2.maps.pre
• preprocess2.maps.post

The target depends list will be

{depend.preprocess.pre},
preprocess2.init,
ditaval-merge,

{depend.preprocess2.maps.pre},
preprocess2.maps,
{depend.preprocess2.maps.post},
{depend.preprocess2.topics.pre},
preprocess2.topics,
{depend.preprocess2.topics.post},

map-clean-map,
clean-preprocess,
copy-files2,
preprocess2.post,
{depend.preprocess.post}

Possible Solution

Allow plug-ins to extend preprocess2 from "safe" points which are likely not be changed in preprocess2.

Potential Alternatives

Plug-ins can fork preprocess2.