Support filenames that end with `+`

[hyunbinseo]

Filenames such as a+.txt are not served because of the following line.

sirv/packages/sirv/index.js

Line 155 in 50b1964

else if (!opts.dotfiles && /(^\.|[\\+|\/+]\.)/.test(name)) return;

1. Why are filenames with +. filtered out?
2. There seems to be a duplicate + in the character-set.

- (^\.|[\\+|\/+]\.)
+ (^\.|[\\+|\/]\.)