Add a way to run commands from a typable command.

[SLUCHABLUB]

I think that it would be nice to have a way of running commands using a typable command. For example :run-command :write or :run-command page_up. I'm unsure what the best name would be; :run, :run-command, :command, and :execute are some alternatives.

The reason why i think this would be nice is that it allows for expansions in the command itself. Theoretical example config:

[keys.insert]
# error: `No command named: '%sh{...'`
C-up = "%sh{echo page_up}"
# error: `No command named: ':%sh{...'`
C-w = ":%sh{echo write}"

C-up = ":run-command %sh{echo page_up}"
C-w = ":run-command %sh{echo :write}"

I couldn't find a way to do this with the current commands, but I may have missed something.

Alternatively, you could allow expansions in the commands themselves.

I will try to create a PR.