fix some corner cases

[daichao1997]

• parseDefaults: Now we can find the last options: string and ignore all text before it
• parseOption: Now we can use both tabs and spaces to separate options from their descriptions. I used to stuck for a long time just because I used tabs (instead of spaces) to align my descriptions.

[coveralls]

Coverage increased (+0.1%) to 88.394% when pulling 3fc2b3d on daichao1997:master into ee0de3b on docopt:master.

[shakefu]

Looks like both nice quality-of-life fixes.