nimgrep: context printing, colorthemes and other

* add context printing (lines after and before a match)
* nimgrep: add exclude/include options
* nimgrep: improve error printing & symlink handling
* nimgrep: rename dangerous `-r` argument
* add a `--newLine` style option for starting matching/context
  lines from a new line
* add color themes: 3 new themes besides default `simple`
* enable printing of multi-line matches with line numbers
* proper display of replace when there was another match replaced at
  the same line / context block
* improve cmdline arguments error reporting