Support popup diff
Support popup `git show`
Refactoring
Update document
Merge pull request #16 from syohex/support-diff

Support diff
Bump up version and update Changes
Reset buffer mode

If show commands executed after diff command, buffer mode is still
diff-mode. This cause invalid highlighting.
Set `no-pager` and `no-ext-diff` option
Merge pull request #17 from syohex/fix-bug

Fix bug
Bump up version and update Changes