'tidy' parameter selection

[jgabry]

As discussed in #161, we should enable parameter selection using vars(...). The version of this now implemented in the feature-tidyselect branch works by allowing the pars argument to be a list of quosures, as returned by dplyr::vars, with a call to tidyselect::vars_select() internally. For example,

x <- example_mcmc_draws()
mcmc_hist(x, pars = vars(-contains("beta"))
mcmc_hist(x, pars = vars(alpha, sigma)) 

The behavior of pars when it is a character vector is unchanged.