ProjectConf: support files/directories/etc... based run

[haya14busa]

cont #47

With #47, reviewdog behavior can be configured by reviewdog.yml.
However, we cannot specify changed files, directories, or whatever with reviewdog.yml.

Example without reviewdog.yml:

$ vint $(git diff --name-only master | grep '.vim') | reviewdog -diff="git diff master" -efm="%f:%l:%c: %m"
autoload/vital/__vital__/Data/String.vim:9:11: Prefer single quoted strings (see Google VimScript Style Guide (Strings))

[haya14busa]

Supporting to pass changed files as an environment variable is simple and useful.

FILES="file1\nfile2\nfile3"

cmd: <lint cmd> $(echo $FILES | grep '.go')