Stricter limits for prefix/target names.
Stricter limits for prefix/target names.

1. Only allow alpha-numerical, and ., -, and _.
2. Require that names for target fields are at least 2 chars;
   this is needed to prevent implicit conversion of tag-types
   by pysam from string to char, which causes failure downstream.
3. Disallow various chars in prefix paths, which may cause
   problems for e.g. Bowtie2.

MikkelSchubert/paleomix,MikkelSchubert/paleomix,MikkelSchubert/paleomix