Add option for preamble before shdo scripts

[idbrii]

I want all of my scripts to explicitly start in the current dirvish working
directory, so I use the example from the doc:
let g:dirvish_shdo_preamble = 'cd {}'

Other users may tend to create scripts with arguments, want their
commands to be inside a function, or output some diagnostic data.

Using the same escaping that is applied on lines since that seems like
the safest thing to do.

[justinmk]

what is motivating this example? :Shdo buffer already automatically sets (:lcd) its local-directory to the Dirvish directory.

[idbrii]

Three reasons:

• Using something like 'autochdir' (Allow autochdir #19) or au BufReadPost * silent! cd %:p:h changes the shdo script cwd. In the latter case, I could block it but would have to block every temp script (*.tmp.bat or $TEMP/*)?
• I'm not comfortable running scripts that rely on cwd. Instead of manually adding the path when doing something destructive like :%Shdo rm {}, this option automatically makes it explicit.
• Gives a nice entry point back to the directory. If I :only, edit the script, finish with it, I can gf on the path since using - brings me to $TEMP (as expected).

[justinmk]

Open to merging this since it's harmless enough, but how about making it a callback? Then the callback can modify the dirvish-generated Shdo script arbitrarily, and also add a "after" script. Something like

g:dirvish_shdo_fmt Callback that accepts a `script:string` arg and returns a string.

Or alternatively, wouldn't it be natural for dirvish#shdo() to emit an event, or set a b:dirvish variable in the Shdo script buffer? Then you can define autocmd(s) that modify the Shdo buffer.

[idbrii]

What do you think about this autocmd version?

I followed tpope/fugitive as an example for invoking the User autocmd and the docs.