Resolves #3.
* Add customizable variable `topspace-active`  to determine when `topspace-mode` mode is active / has any effect on buffer.
It can be set to t, nil, or a predicate function. This is useful in particular when `global-topspace-mode` is enabled but you want
`topspace-mode` to be inactive in certain buffers or in any specific
circumstance.  When inactive, `topspace-mode` will still technically be on,
but will be effectively off and have no effect on the buffer.

* Allow `topspace-autocenter-buffers` to be either t, nil, or a predicate function (before it could only be t or nil)

Moving to automatic changelog generator after this, but some legacy developement history does not follow the correct API for the automatic changelog, so this legacy changelog file will better show version changes made before this commit.

- rename topspace--eval-choice-p to topspace--eval-choice
- rename topspace--put to topspace--draw
- Fix flycheck warnings
- Update version number

- Fix indentation

- Make clarification in docstring

- Update window-configuration-change

- Preserve previous column position when correcting the cursor from
  scrolling past window-end

- Make private how many lines away from `window-end` cursor can get

- Fix issue from PR #10 that broke ability to scroll down when in terminal:
  Resolved by making the line height calculations on integers instead of
  floating-point when in terminal.

- Fix minor issue from PR #10 caused when scrolling with mouse:
  When mouse scrolled the buffer all the way down it made the top space
  unexpectedly jump. This was resolved by lowering topspace command hook
  priorities such that the hooks are run sooner in the list.

- PR #10 introduced a bug where scrolling doesn't working as expected
  when Emacs is run in terminal. The fix for this in the previous commit
  was not sufficient. This commit fixes the source of the problem,
  which was that floats were being passed to
  scroll-up/scroll-down from the added advice functions, but that
  seems to be invalid when in terminal. So now only integers are being passed
  to scroll-up/scroll-down when in terminal.

- Remove extra keywords from slogan to make it more concise
- Add usage section & reorder other sections
- Add link to overlay term

- Minor correction/rewording

Update GIF to show the topspace-empty-line-indicator in the left fringe

Increase text size in GIF

Increase text size of GIF

- #11 caused a bug where is was not possible to scroll anymore if
  topspace is disabled in the current buffer. This resolves that issue.

- #11 caused a bug with mouse scrolling - when scrolling down
  it would move buffer suddenly all the way to the bottom
  in some cases. This resolves that issue.

- Prevent a "Beginning of buffer" error message from appearing in the
  *Messages* buffer when scrolling above the top line.
- No longer modify `mwheel-scroll-down-function`, because before it was
  modified in order to prevent a scrolling bug, but that bug is avoided
  now by preventing the "Beginning of buffer" error from occurring.

- The buffer would get scrolled up too far by one line when scrolling
  text up in the following condition: top space was present before
  scrolling but not present after, and the top line in the buffer should
  be at the very top of the window after (but the bug made the second line
  be at the top of the window). This is resolved here.
- The technical reason for the bug was that 0.0 would be passed to `scroll-up`
  after `topspace--filter-args-scroll-up`, assuming that `scroll-up` would not
  scroll in this scenario, however it turns out that if
  `scroll-up`/`scroll-down` are passed a zero-valued float they will
  still scroll by one line.