Add string.escapepattern function

[ghost]

Using filenames with string.match and string.find, etc. is problematic because they take Lua patterns as an argument where ( ) . % + - * ? [ ] ^ $ are special characters.

This pull request adds a string.escapepattern function that escapes strings to be used with patterns.

[samsinsane]

Can you add tests for this, not only for positive cases but also some negative cases.