Support trailing whitespace & floating point values with Brendan Gregg collapsed stack format

So I have some custom tool that generates data in a output similar to Brendan Gregg's stack format, that works with flamegraph.pl, but is throwing issues with my custom file.

I found there were a few different issues in my file:
- a trailing newline in the file causing an error
- my values weren't integers
- some of my values were negative

Not sure if we could raise a better error message, for those?

Maybe it'd also be useful mentioning that in the docs, I am happy to make a PR, if there is a way to do that in docs?