Allow fixing (editing) invalid commit message interactively

Before, in interactive mode, an invalid commit message would have
resulted in the user being asked:
  Continue anyway? [yes/no]
Now there's also the option of editing the message:
  Continue anyway? [yes/no/edit]

Also, now if the user gives an invalid reply i.e. types 'ues' the
script won't exit immediately but prints an error message and waits
for another response.

The option of editing the commit message is given only if the input
(i.e. the commit message) is passed in as a regular file. This is
always the case when lint-commit-msg is used in commit-msg hook.
Even if the user runs
  git commit -m "..."
the commit message is written into a temporary file that is passed
to commit-msg hook (and then to lint-commit-msg).

So, in which case the user is prevented from selecting 'edit'? When
the commit message is fed into lint-commit-msg via stdin. However,
this can only happen when lint-commit-msg is run standalone... and
in that case the interactive mode is not used... unless LCM_INTERACTIVE
is set to 'always'. So, that's the corner case when the user should
not be able to edit the commit message.

Other possibility is a file without 'write' permission
  LCM_INTERACTIVE=always lint-commit-msg not-writable-file.txt
in which case trying to edit the file would be pointless.

README.md

@@ -19,7 +19,7 @@ ERROR: commit message not properly formatted
 - line 1: subject not capitalized
   add first version of REST client and instructions how to use it with HTTPS
 
-Continue anyway? [yes/no] no
+Continue anyway? [yes/no/edit] no
 Aborting commit!
 Commit message saved in .git/lint-commit-msg.MSG
 $
@@ -160,6 +160,9 @@ The following sections describes each configuration variable and
 
 The variables above will also accept values `true` (alias for `always`) and `false` (alias for `never`).
 
+`lint-commit-msg` also inspects `GIT_EDITOR`, `VISUAL`, and `EDITOR` (in this order) for a text editor
+to launch if the user wishes to edit an invalid commit message interactively.
+
 ### Modifying rules
 
 | Environment variable | Description | Default value |