support opening and closing delimiters

[davidchambers]

Commit message:

For optimal Transcribe compatibility, conflicts with Markdown syntax must be avoided. Markdown uses GREATER-THAN SIGN (>) at the beginning of each line of a <blockquote>. This conflicts with that character's use to signify doctest input, as in the following example:

//# K :: a -> b -> a
//.
//. This is a paragraph.
//.
//. > This is a block quotation.
//. > It should not be evaluated!
//.
//. ```javascript
//. > K (true) (42)
//. true
//. ```

This commit adds --opening-delimiter and --closing-delimiter options to the command-line interface, enabling the use of Markdown's <blockquote> syntax in source files with delimited doctest blocks.

The following options would resolve the conflict in the example above:

$ doctest --prefix . \
.         --opening-delimiter '```javascript' \
.         --closing-delimiter '```' \
.         -- path/to/file.js

See sanctuary-js/sanctuary#600 for the motivation for this feature.

I don't think processLine requires careful review as I spent several hours tightening it after getting it to work. I would, though, appreciate a review of the additions to the command-line interface. Are you happy with the names and descriptions of --opening-delimiter and --closing-delimiter?

/cc @Bradcomp

[danse]

Looks good! 😎