minwidth now accepts a percentage of time with %

[matthew-olson-intel]

Refactoring minwidth to additionally accept a percentage of total time instead of a pixel value. The disadvantage here is that GetOptions has to accept minwidth as a string, and we have to do the string->float conversion ourselves; in this case, I chose to use a regex to find digits and periods, and print the usage string if it couldn't be persuaded to be a float.

[matthew-olson-intel]

Should fix #315.

[brendangregg]

thanks!