Allow custom column names for `post_treatment` variable in DiD for clarity

[roesta07]

Allow the user to use whatever column name they want for 'post_treatment' variable while constructing DifferenceInDifferences object with new parameter post_treatment_variable_name . Also setting its default value to 'post_treatment' so that it does not break previously written codes.
@drbenvincent As discussed more in issue no #390

[drbenvincent]

Closed by #515