Add short command aliases

[VAWVAW]

fixes #535

[ulyssa]

Unfortunately, a lot of these are short aliases that could reasonably be added in the future, and that people might want to use because of how common they are in Vim, like :s to make replacements on the current line, or :r to read in an external file into the message bar.

I think the better approach is probably to allow people to specify their own aliases in their configuration file. I've made ulyssa/modalkit#220 so that that can be done here in iamb.