- closes #281

Clarify Readme.

Update minirc to Plugin* commands with #begin and #end

Use svg instead of png to get better image quality

The command `silent pedit ...` was causing trouble if the user had `set
hidden`.  Now we `bdelete` the changelog buffer before editing it anew.

The variable is script local to autoload/vundle/scripts.vim since #468.

These variables only occur in one file each.  By making them script local
variables this is "documented" in the code.  At the same time the global
namespace is polluted less.

Changed:
    g:bundle_names          -> s:bundle_names
    g:vundle_last_status    -> s:last_status
    g:vundle_log_file       -> s:log_file
    g:vundle_view           -> s:view

If it was loaded, unload the log buffer before editing it.  Otherwise the
editing command can "hang" if the user has `set hidden`.

This problem was originally discovered with the VundleChangelog command and
the analogous fix was applied in 7d9b10.  See github issue #468 for more.

All global variables that are not part of the public API (mentioned in the
documentation) are turned into autoload variables.  This is intended to give
all global variables defined by Vundle.vim a common prefix.  The variable
g:default_git_proto is part of the public API and is therefor not changed.
This is the only exception.

Changed:
   g:bundle_dir            -> vundle#bundle_dir
   g:bundles               -> vundle#bundles
   g:updated_bundles       -> vundle#updated_bundles
   g:vundle_lazy_load      -> vundle#lazy_load
   g:vundle_log            -> vundle#log
Unchanged:
   g:default_git_proto

The setting of the default values for the autoload variables is moved out of
any function.  One reason being that these settings do not depend on the
argument of the function.  The second being that Vim will source the autoload
script if an undefined autoload variable is referenced and the file is
expected to define the variable (see :help autoload).

Refactor global variables to autoload variables

Plugins' names which contain '../', '$HOME', '%:h:h', '..\', etc.
can be dangerous.
Use a sensible whitelist for plugin names and prevent its expansion.

Prevent insecure plugin names.