Created module for executing diff commands.

Returns the necessary additions and deletions to apply in order to
transform the from-file into the to-file.
Added function to align the file contents.

Uses the change set produced by the diff output, i.e. by calling the
diff.exec() function to add empty lines as needed. The file contents
can then be iterated through simultaneously in order to produce a
side-by-side diff.
Return new set of changes based off alignment.

Since the alignment of the file contents creates a unified
line-numbering scheme, there is no need to store both `line` and `rel`
fields anymore.
Updated the doc comments in the function header.

Also renamed the callback parameter from `done` to `next`. This was an
artifact of the original thought to have the file contents arrays be
mutated, so no return values were actually needed.
Created module to read all lines from a file.

Returns an array of strings, where each element represents a line.
Basically wraps the builtin 'readline' module.