Update paper reference in README
Add exact kinematic hard factors to README
Add exact_kinematics to README
Choose NLO hard factors based on exact kinematics

As of this commit, specifying nlo as a hard factor specification on
the command line will choose h1qqexact and h1ggexact instead of the
usual qq and gg hard factors if exact_kinematics is set.
Fix implementation of split()

This commit fixes a bug where split(string, chars, n) would split
the string into one too many pieces if one of the chars was the
first character of the string.
Enable parsing labeled hard factors

This allows the program to accept a hard factor with a label. In
this commit, instead of looking for a preset pattern for hard factors,
we try parsing each unrecognized argument as a hard factor specification.