Mechanism for (un)shielding symbols from substitution

[brodieG]

Implement as an additional arg, e.g. shield.with='.', which then would allow things like:

recsub(quote(f(a + .(b)))

where b would be shielded from sub.

Potentially could add a second argument to allow skipping matching symbols, e.g. in a situation like:

Species <- quote(Species == 'versicolor')
Sepal.Width <- runif(150)
b <- quote(Sepal.Width > 2)
recsub(quote(iris[.(Species, 1) & b,], iris))

where we do not want the Species in iris to prevent expansion, but we do want normal behavior for everything else (hmm, this is not a very good example).