Fix escaping in Linux filter_region.

[daviddonna]

Text that gets filtered through a command (e.g. with :'<,'>!tr x y) is interpolated between double quotes in a command string. If the text in question contained double quotes, backslashes, or dollar signs (which indicate variable or shell interpolation), the shell would parse them as special characters, usually failing.

This patch adds a bash_escape_double_quoted_text function that escapes those characters in the text getting filtered, so that the shell treats them as plain.