wp-cli support with an htaccess importer

Update the create_redirect() method to return WP_Error if the redirect already exists. Duplicate redirects don't make sense.
Bump working version number
Allow for htaccess redirects that specify status codes in WP-CLI command import-htaccess
Merge pull request #31 from tlovett1/htaccess-importer

wp-cli support with an htaccess importer
Any single quotes, at least, in `$_SERVER['REQUEST_URI']` are slashed, unlash them before matching

Manual cherry pick of https://github.com/simonwheatley/Safe-Redirect-Manager/commit/809c565e4e63d53be86c4aac7ec4c8edc6bda3d1

Closes #29
Allow redirects to be bulk deleted from the Manage Redirects view

Closes #23
A new method for getting a set of redirects, optionally limited by some criteria
A wp-cli subcommand for listing a table of all redirects

See #32
Update create_redirect() method to accept more arguments, and always return WP_Error objects on failure
New wp-cli subcommands for creating and deleting redirects, and updating cache

See #32
Update readme with notes on #32
Update update_redirect_cache() to use our new get_redirects() method.
Tag 1.6
Update readme and version, include Slovak language files
Apply prope sanitization to avoid stripping characters that shouldn't actually be stripped
Merge pull request #34 from Automattic/proper-sanitize

Apply prope sanitization to avoid stripping characters that shouldn't ac...
Bump working version to 1.6.1-alpha
Check if get_post is empty instead of false in filter_admin_title [fixes #35]
Query for redirects in sets of 50, to avoid massive queries when we have lots of redirects
rm action, method doesn't exist