Add support for CSV comments

## Background
CSV supports comments. Each line that begins with a comment character ('#' by default) is skipped.

## Feature
Add a command-line option to specify comment character. An empty string can also be specified, either as `''` or as per #1319 .

## Example [Optional]
```
spark-submit ... --csv-comment #
```

## Proposed Solution [Optional]
Use `.option("comment", char)` to specify the comment character (default="#"). If it is `null`, comments are disabled.