Allow special symbols to be mapped to whitespace with a warning

Conflicts:

	lm/builder/corpus_count.cc
	lm/builder/lmplz_main.cc
	lm/builder/pipeline.hh
-h help option for lmplz
Vocabulary pad

Conflicts:

	lm/builder/interpolate.cc
	lm/builder/interpolate.hh
	lm/builder/lmplz_main.cc
	lm/builder/pipeline.cc
	lm/builder/pipeline.hh