add '$ -e', and 'show-source -e'.

evaluates the argument to the pry command as an expression and returns
the source for the return value of the expression. i need to add tests
as well as update changelog.

closes #1184
update CHANGELOG.md for #1185
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
rewrite test on top of `ReplTester`.
Merge pull request #1185 from johnny5-/showsource-e

add '$ -e', and 'show-source -e'.