Revisit progress bar feature for write functions.

[johncassil]

This was discussed in #277 and it seems that competing priorities (making writing faster) put it to bed.

However, for the sake of consistency and psychological well-being, users expect that when they are writing files out, they would get an ETA just like when they load it in.

I have trained several new R users on read_csv, and they have all expressed confusion / bafflement that there is no similar progress bar on writing a file back out. This leaves them to depend on watching the stop button in the RStudio IDE to see when it disappears, waiting for the > in the terminal to appear, or my favorite, using beepr:beep() for audial feedback when the write process is finished.