Load all fixture grammars dynamically

This way the build doesn't take forever any time a single grammar has 
been regenerated.

script/regenerate-fixtures

@@ -2,6 +2,8 @@
 
 set -e
 
+cargo build --release
+
 root_dir=$PWD
 tree_sitter=${root_dir}/target/release/tree-sitter
 grammars_dir=${root_dir}/test/fixtures/grammars
@@ -19,6 +21,10 @@ grammar_names=(
   rust
 )
 
+if [[ "$#" > 0 ]]; then
+  grammar_names=($1)
+fi
+
 for grammar_name in "${grammar_names[@]}"; do
   echo "Regenerating ${grammar_name} parser"
   cd ${grammars_dir}/${grammar_name}