feat(write_bib): escape & as much as possible (closes #2335)

[atusy]

In write_bib, apply gsub('(?<!\\\\)&', '\\\\&', x, perl = TRUE) to escape unespcaped ampersands.

Note that released implementation partially applies the escape.
Instead, I apply the escape as later as possible so that the escape happens widely.

[yihui]

Looks good. Thank you!

Instead, I apply the escape as later as possible so that the escape happens widely.

I think that's a very good idea.