If the exists('*exepath') evaluates to false vim was returning this
error:
Error detected while processing function
<SNR>10_fmtAutosave..jsonnet#Format..jsonnet#CheckBinPath:
line 6:
E121: Undefined variable: binPath
Added documentation, a vim command, a user callable function and an
autocommand that gets executed at every save.
The defaults are hight configurable with options that are also
documented in this change.
