Allow on-the-fly vcov adjustment

[grantmcdermott]

Closes #34.

Examples.

pkgload::load_all("~/Documents/Projects/ggfixest/")
#> ℹ Loading ggfixest
#> Loading required package: ggplot2
#> 
#> Loading required package: fixest

# ggiplot ----

# default is clustered by id
est_did = feols(y ~ x1 + i(period, treat, 5) | id + period, base_did)

# focusing here on period 6 
ggiplot(est_did, keep = "6", zero = FALSE, vcov = "iid") +
    geom_hline(yintercept = confint(est_did, vcov = "iid")[[1]][6], lty = 2) + 
    geom_hline(yintercept = confint(est_did, vcov = "iid")[[2]][6], lty = 2)
#> Warning: Removed 1 rows containing missing values (`geom_vline()`).

# ggcoefplot ----

est = feols(Petal.Length ~ Petal.Width + Sepal.Length + Sepal.Width | Species, iris)

ggcoefplot(est, zero = FALSE, vcov = "iid") +
    geom_hline(yintercept = confint(est, vcov = "iid")[[1]], lty = 2) + 
    geom_hline(yintercept = confint(est, vcov = "iid")[[2]], lty = 2)

^{Created on 2023-12-12 with reprex v2.0.2}

Two asides:

1. I don't think the latter option works with the base coefplot option. Need to raise an upstream bug issue.
2. I have decided against passing other potential arguments through ... to summary.fixest. Just limiting it to these vcov adjustment options, otherwise we risk silent errors.