* Fix --headerless option of to-csv and to-tsv
Before to-csv --headerless split the "headerfull" output on lines,
skipped the first, and then concatenated them together. That meant
that all remaining output would be put on the same line, without
any delimiter, making it unusable. Now we replace the range of the
first line with the empty string, maintaining the rest of the
output unchanged.
* Remove extra space in indentation of to_delimited_data
* Add --separator <string> argument to to-csv
This functionaliy has been present before, but wasn't exposed
to the command.
This improves incremental build time when working on what was previously
the root package. For example, previously all plugins would be rebuilt
with a change to `src/commands/classified/external.rs`, but now only
`nu-cli` will have to be rebuilt (and anything that depends on it).
