SourceMap: Fix relative paths for Windows. (#242)
It required converting all paths to lower-case,
the rest of the implementation was there.
This lower-casing is not locale-aware, as NTFS is
all case-sensitive when it goes beyond ASCII.
Interface: Use input filename to contruct the
default value of output.
Merge pull request #441 from am11/issue-242

SourceMap: Fix relative paths for Windows (#242)