Add top margin option. Set options to wrap text. Fix doc.

[PhilRunninger]

The changes are pretty well documented in the commit messages. In summary:

• Updated the documentation to remove obsolete reference to dictionary
• Fixed formatting of the plugin's name in the help doc.
• Added an option to insert a specified number of blank lines above each slide.
• Appended blank lines after each slide to push the ~ lines off the window.
• Set 4 vim options to enable wrapping of text.

[sotte]

Thanks for the contributions!

Also, can you add an example in the examples/ folder which demonstrates the feature. You can use vim's modeline to set the presenting_top_margin.

Now that I'm thinking about it I'm wondering if "add top margins" is really what we want, or if it's rather a "center vertically" feature. What do you think?

[PhilRunninger]

Regarding the example, I may be missing something, but isn't the modeline used to set options, not variables? Vim is complaining that presenting_top_margin is an unknown option.

<!--
vim:tw=40:ft=markdown:presenting_top_margin=5
-->

Centering vertically could be challenging if lines start wrapping. We can count the lines of text in the array and make a calculation that way, but if a line displays on more than one, the text will be too low on the screen. Personally, I like seeing the slides with a consistent top margin, à la Powerpoint.

[sotte]

Hey Phil,

you're totally right about the modeline and variables. For whatever reason I thought you can do it. So no example file needed.

Centering vertically: maybe it's too much trouble. I'll merge it and we can change it later (if someone want to do that :)).

Thanks for the contributions!