Delete the file, not some random lines

Deleting a file is a function call. Never saw any bugs related to this.
Will have to see if actually deleting the file causes issues.
Use noremap to prevent mapping breaks

If someone remaps : or <CR> or something else, this map should still
work.
Use grepprg for AsyncGrep

If someone's customizing their grep command, we should use that. On
Windows, we should be using findstr unless configured otherwise.

To get the exact same behavior as before, use:
    let &grepprg = 'grep --line-number --with-filename'
or (as the help for grepprg suggests):
    set grepprg=grep\ -nH

(They're equivalent.)
Accept $* placeholders for grepprg and makeprg

grepprg and makeprg allow optional $* placeholders for the location of
arguments. If omitted, the arguments are appended to the prg command.
Emulate this behavior for AsyncGrep and AsyncMake.
Add support for a simple statusline segment

Adds asynccommand#statusline() that has configurable text to display the
number of background jobs. Does not automatically modify 'statusline'.
Add a pending command listing

Add AsyncPending to make it possible to find in-progress log files.
Helpful to see the output of a pending compile command or to figure out
what you have running in the background.

Only uses existing information, and only some handlers add a title to
their env Dictionary. May be more useful if there's more information
here (and it might be nice for all of the output windows to have useful
titles).
Add documentation for other commands

I originally omitted docs for these commands because I considered them
examples and wanted to move them to separate plugins, but since they all
replace built-in vim functionality I'm keeping them as part of the core
plugin.
Pass escape sequences to prg_command

Escape escape-sequences so they can be passed to prg_command. Can't use
shellescape because it escapes too much (doubles quotes and wraps whole
expression in quotes). We just want the user's escapes to be passed to
grep, make, etc...

Fixes #21
Fix :AsyncPending when called from non-modifiable buffer
Merge pull request #26 from mah/fixAsyncPending

Fix :AsyncPending when called from non-modifiable buffer

Ensure pending buffer is modifiable before modifying it. Since you can't
change what's displayed (no cancelling), make it not modifiable.
Fix #25: Add a %d for quickfix title

Instead of %s (which isn't used), use %d to get the number items in the
quickfix/locationlist so you can see at a glance the magnitude of the
results.

Add %d where it was missing.

Add documentation.

You may not see results when using fancy statuslines like powerline. You
can test what it would display:
	:echo w:quickfix_title
Adding pending file autoread and externally-accessible command formatting.
Merge remote-tracking branch 'origin/master'

Conflicts:
	plugin/asynccommand.vim