For environment 'localhost', this class reads the list of (feature, treatment) pairs
from a file $HOME/.splits, and returns an instance of HardcodedFeatureTreatmentsSplitClient.
$HOME/.splits has two columns separated by whitespace. First column is the name of the feature,
the second column is the name of the treatment to always return for the corresponding feature.
Empty lines or lines without exactly two columns are ignored. Lines that start with '#' will be
ignored as comments.
If $HOME/.splits is not found, all features are assumed off.