Lookatme

Makes the user aware of any new, not-already-manually-approved extensions
that the source markdown wants to load.

Also checks all loaded extensions for a `user_warnings` function that returns
a list of messages to display to the user before using the extension.

Adds new `-i`, `--safe`, and `--no-ext-warn` command-line arguments.

| type | ticket | description |
|--------:|-----------------------------------------------------------|---------------------------------------------------------------|
| feature | [109](https://github.com/d0c-s4vage/lookatme/issues/109) | More robust extension handling, does not auto-load by default |

Removes copyrighted image, adds `-e` and `--exts` options

| type | ticket | description |
|--------:|-----------------------------------------------------------|-------------------------------------|
| feature | [103](https://github.com/d0c-s4vage/lookatme/issues/103) | Adds ability to pre-load extensions |
| bug | [107](https://github.com/d0c-s4vage/lookatme/issues/107) | Removes copyrighted image |

Adds customizable slide margins and paddings

| type | ticket | description |
|--------:|---------------------------------------------------------|---------------------------------------------|
| feature | [95](https://github.com/d0c-s4vage/lookatme/issues/95) | Make slide padding and margins customizable |

Changes `render_text` to be able to support widgets instead of only text
attributes. This is a breaking change to the interface of `render_text`.
Plugins that relied on `render_text` will need to be updated.

| type | ticket | description |
|--------:|-----------------------------------------------------------|----------------------------------------------------------------|
| feature | [101](https://github.com/d0c-s4vage/lookatme/issues/101) | Inline render functions should be able to return urwid Widgets |

Vertical scrolling on overflowed slides now works, added `--single` and `--one`
command-line options and hrule rendering.

| type | ticket | description |
|--------:|---------------------------------------------------------|---------------------------------------------------------|
| feature | [96](https://github.com/d0c-s4vage/lookatme/issues/96) | Add new cmd-line option to force single slide rendering |
| feature | [91](https://github.com/d0c-s4vage/lookatme/issues/91) | Single slide |
| feature | [29](https://github.com/d0c-s4vage/lookatme/issues/29) | Slides should be vertically scrollable |