config: Rename Replace WriteMode variant to Backup

[kamalmarhubi]

The new variant name more closely matches its purpose. The rustfmt
binary is modified to accept --write-mode replace as a synonym for the
new name.

[kamalmarhubi]

r? @marcusklaas

[kamalmarhubi]

NB the change in rustfmt binary does not pick up an explicit write-mode in the rustfmt.toml file. Projects with write_mode = "replace" in their project file will see a decode failure. I think this is acceptable, as it's not really a setting that belongs in the project file anyway (cf #871). If you prefer, I can implement rustc_serialize::Decodable directly or mess with the macros to make it work.

[marcusklaas]

I don't feel confident enough about this to merge it. What do you think, @nrc?

[nrc]

I'm not really sure about this change, since backup is a side effect of the main action (which is overwriting). I guess the ideal would be OverwriteWithBackup, but that is unpleasantly verbose.

While I agree that the replace/overwrite distinction is vague and annoying, I don't think it is worth changing unless we have a really superior alternative.