Add asymptotic confidence intervals for average treatment effect for linear regression with effect modifiers

Refer to #335. DoWhy defaults to bootstrap confidence interval. It will be good to implement a computationally efficient confidence interval method.

With effect modifiers, the average treatment effect is a linear combination of parameters. For example, for y=a + b1t + b2t.x1 where x1 is the effect modifier, the ATE of t on y is b1+b2.mean(x1). Both b1 and b2 coefficients are random variables and so is mean(x1), so need to compute the standard error of the combined quantity.

This is a good start, for anyone who would like to contribute: https://scholar.princeton.edu/sites/default/files/jmummolo/files/interaction_models_jm.pdf