Deleting bookmarks with invalid names is impossible

[techwizrd]

Although it is possible to add bookmarks with invalid names as per Issue #10, there is no way to delete them. This leaves the user with no way to fix their situation other than manually editing the ~/.sdirs file.

Operating System: Ubuntu 13.04 "Raring Ringtail"
Fish Shell: fish, version 2.0.0 (Compiled from Git version fish-shell/fish-shell@a62ebc9)
Fishmarks version: 5f4771d
Steps to Reproduce:
The terminal session below demonstrates how to reproduce the error.

[~]$ mkdir test
[~]$ cd test
[~/test]$ s test~dir
set: Invalid character “~” in variable name. Only alphanumerical characters and underscores are valid in a variable name.
/home/kunal/.fishmarks/marks.fish (line 157): set -x DIR_test~dir "$HOME/test"
                                                      ^
in . (source) call of file “-”,
        called on line 140 of file “/home/kunal/.fishmarks/marks.fish”,

in function “_update_completions”,
        called on line 50 of file “/home/kunal/.fishmarks/marks.fish”,

in function “save_bookmark”,
        called on line 1 of file “-”,
        with parameter list “test~dir”

in function “s”,
        called on standard input,
        with parameter list “test~dir”
[~/test]$ d test~dir
set: Invalid character “~” in variable name. Only alphanumerical characters and underscores are valid in a variable name.
/home/kunal/Projects/fishmarks/marks.fish (line 143): set -x DIR_test~dir "$HOME/test"
                                                      ^
in . (source) call of file “-”,
        called on line 129 of file “/home/kunal/.fishmarks/marks.fish”,

in function “_valid_bookmark”,
        called on line 92 of file “/home/kunal/.fishmarks/marks.fish”,
        with parameter list “test~dir”

in function “delete_bookmark”,
        called on line 1 of file “-”,
        with parameter list “test~dir”

in function “d”,
        called on standard input,
        with parameter list “test~dir”

ERROR: bookmark 'test~dir' does not exist