searcher: always strip BOM

This fixes a bug where a BOM prefix was included. While this was somewhat
intentional in order to have a faithful "UTF8 passthru" option, in
practice, this causes problems such as breaking patterns like `^` in a
really non-obvious way.

The actual fix was to add a new API to encoding_rs_io, which this commit
brings in.

Fixes #1163

grep-searcher/src/searcher/mod.rs

@@ -307,6 +307,7 @@ impl SearcherBuilder {
         decode_builder
             .encoding(self.config.encoding.as_ref().map(|e| e.0))
             .utf8_passthru(true)
+            .strip_bom(true)
             .bom_override(true);
         Searcher {
             config: config,