User-facing tinytheme_get() function + restore plot region after theme

[grantmcdermott]

Closes #629

Follow-up on two good points made by @bastistician in this comment, after I'd already closed the issue.

1. Users can now invoke the tinytheme_get() convenience function to grab the name of the current theme.

pkgload::load_all("~/Documents/Projects/tinyplot")
#> ℹ Loading tinyplot
tinytheme("classic")
tinytheme_get()
#> [1] "classic"
tinytheme()
tinytheme_get()
#> [1] "default"

2. type_v/hline and co. correctly reach the axis after a text() call. This ended up being slightly trickier to catch/and plug. The underlying issue is that a theme's margins are applied through a before.plot.new hook, so rolling the theme back on exit left par describing a different plot region than the one that was drawn into. Meanwhile, base R only recomputes the device clipping rectangle when xpd changes, so anything drawn after an intervening xpd toggle (e.g. text()-based, but also box(), etc.) was clipped short.

pkgload::load_all("~/Documents/Projects/tinyplot")
#> ℹ Loading tinyplot
plt(0, 0, theme = "classic")
plt_add(par("usr")[1], y = 0.3, type = type_text(labels = "foo", xpd = NA, pos = 2))
plt_add(type = type_hline(0.3))