Add .id argument to bind_rows

This makes row_binds append an identifier column when the `.id` argument is supplied.

This is useful for the common use case of combining data frames to be used for example by ggplot2. Best used with lowliner :-)

```
fits <- list(lm(disp ~ cyl, mtcars), lm(disp ~ mpg, mtcars)) %>%
  map(broom::tidy) %>%
  bind_rows(.id = "model")

## > + Source: local data frame [4 x 6]
##
##          term estimate std.error statistic    p.value      id
## 1 (Intercept) -156.609   35.1805   -4.4516 1.0902e-04 model 1
## 2         cyl   62.599    5.4693   11.4455 1.8028e-12 model 1
## 3 (Intercept)  580.884   41.7401   13.9167 1.2634e-14 model 2
## 4         mpg  -17.429    1.9925   -8.7472 9.3803e-10 model 2
```