Improve line-jumping on show old / new source.

diffscuss-mode now appropriately figures out which line to jump to and
goes there, for both old and new source, in the cases where:

* The cursor is on a line starting with '-' (an old line)

* The cursor is on a line starting with '+' (a new line)

* The cursor is on a line starting with ' ' (a common line)

* The cursor is on a line starting with @@ (a hunk line)

Still need to deal with cases where the cursor is above a hunk line
(in the metadata) and maybe where the cursor is on a '\ No newline'
line.
Add a blank comment line at end of new comment.
Position cursor correctly after replying to a comment.
Add support for file-level comments.

If you're at the very top of the file and hit C-c C-c, make a file
level comment, above everything else.

Closes #3.
Handle C-c C-c intelligently in all contexts.

I.e.:

* If at position 1 in the file, add a file-level comment.

* If in a comment, reply to it.

* If anywhere in a diff header, start a comment just past it.

* Otherwise, add a comment for the current line.

Closes #5.
Insert ISO 8601 date on new comment.

Closes #4.
Make the executable a variable in diffscuss mode.

Closes #6.
Make diffscuss-open-line degrade to open-line on non-diffscuss lines.
Use git config to find author name, rather than login name.

Closes #9.
Insert email on new comments.

Defaulted to git config user.email.

Closes #10.
Provide script to create diffscuss file from git revision range.

Closes #1.
Make new file comments jump to end of thread.

For consistency in ordering, when there's an existing file comment,
jump to the end of the thread when inserting a new file comment.
Update README.md to reflect the current state of the union.
Fix README.md formatting.
Update walker.py to reflect changes to diffscuss format.

Specifically:

* Allow empty header lines

* Allow empty body lines

* Allow empty header lines before an author header

* Allow comments at the top of the file
Recognize diff headers in walk.

To make it easier to work with the data stream.
Add script to find the file and line for local source.
Consult find-local-source.py from emacs mode to jump to local source.

Error handling is probably a little suspect.
Update README.md with method for jumping to local source.
Add keys for jumping to next / previous comment in emacs.

Closes #8.