Added autocompletion for filetypes with !

Now, :Neoformat! will complete available filetypes
And :Neoformat will still complete available formatters

Added stdin option for formatters (sbdchd#16)

Before:

buffer --> /tmp/file --> formatter --> /tmp/file --> buffer

Now (with 'stdin': 1):

buffer --> formatter --> buffer

Removed job_control as it isn't necessary

Adjusted formatters that support stdin to have stdin option set to 1.

Added check to make sure buffer is modifiable

Check &modifiable before starting neoformat

Also added logging to cmd definition

added fix for composite filetypes

Now, instead of spitting an error message into (n)vim's messages,
the composite filetype is split and the first filetype is used.
e.g. `javascript.jsx` --> `javascript`

update readme with vim compatibility

added docs 🐋