Don't ignore non-trivial lines with no indentation

[jberger]

When using the removeIndentation setting globally and including whole
files, I notice that an odd thing happens. All code that is indented is
outdented by one level! On investigation, this is caused by ignoring
lines that begin without whitespace when computing the indentation level
to consider the base.

With this modified regex you still get the same result, ie a string of
leading whitespace, however a line without leading whitespace still
matches and the length of that result is 0. The trivial case of an
entirely blank line is also handled by including a zero-width forward
lookahead assertion that some non-whitespace character must occur after
the leading optional indentation in order for that line to be included
in the match.

[jberger]

If this is confusing, let me demonstrate:

one
  two
    three

currently becomes

one
two
  three

but with my patch it does the right thing, even if the full text is indented but contains an empty line

  two

    three

becomes

two

  three