Fix typo in show_source.rb

1. delete EXPRESSION from usage in BANNER, because user can use EXPRESSION only when -e option is given.

2. add `show-source Pry.foo -e` example to BANNER.

3. rename parameter of options method from `opts` to `opt`, because in other commands `opt` is used.
Add test to show_source command with -e option.
Merge pull request #1 from yui-knk/johnny5/showsource-e

Fix typo in show_source.rb