Multifile prototype1

Fix for hmlgrep: open and process files separately rathar than 'cat'
all the input files for two reasons.
* When a topmost entry of second file doesn't have
  a header, it will be treated as continuation of last entry of first
  file, but it should be separate record.
* When using --count option, it is desireble to see number of matched
  entries separately for each of files. (like grep)
If the old behabior is preferable, users can just use cat command.

Still need to fix:
* hmlgrep & pymlgrep: doesn't print filename when --count is given
* amlgrep: Exits with error when not all files have matches.
         (looks it's using AND logic...should be OR)
code clean-up
Merge branch 'master' of github.com:ryo1kato/mlr-grep into multifile
Show filename with --count option option for multiple files

Just like `grep --count`