Do not add normal mode mappings for Vim.

Closes #35
Improve mapping for Vim, using <Plug> maps>

Add redirection through <Plug>SparkupExecute and <Plug>SparkupNext and
check if mappings are defined for them before assigning default maps.
Fix doc installation in "vim" Makefile task.

Copy README to 'doc' subdirectory and mark the header with asterisks.
The latter appears to be required for `:helptags` to pick it up.
Use `inoremap` for calling Sparkup{,Next}().

This will make the mapping work properly with e.g.
`noremap : .`.

See https://github.com/rstacruz/sparkup/pull/40#issuecomment-1202903
Use `return` instead of `finish` in a function
Use full path for s:sparkup

When determining `s:sparkup` the whole path should get used.
`findfile()` might return a relative path, so we use `fnamemodify(..,
':p')` to transform it into an absolute path.
Mention sparkup.py besides sparkup in the error message
Unset s:sparkup in case of error

Besides allowing for a re-detection on next invocation, it will prevent
and error from trying to execute an empty command.