Search extra patches from given directory then apply it during build (#91)

```
$ env PYTHON_PATCH_PATH="./patches" PYTHON_PATCH_OPTS="-p1" pyenv install 2.7.6
```

plugins/python-build/bin/python-build

@@ -162,17 +162,24 @@ install_package_using() {
 
 apply_patches() {
   local package_name="$1"
-  local patches_path="${PYTHON_BUILD_ROOT}/share/python-build/patches/${DEFINITION_PATH##*/}"
 
   # Support PYTHON_PATCH_STRIP, etc.
   local package_var_name="$(capitalize "${package_name%%-*}")"
+  local PACKAGE_PATCH="${package_var_name}_PATCH"
+  local PACKAGE_PATCH_PATH="${package_var_name}_PATCH_PATH"
   local PACKAGE_PATCH_STRIP="${package_var_name}_PATCH_STRIP"
+  local PACKAGE_PATCH_OPTS="${package_var_name}_PATCH_OPTS"
+  local PACKAGE_PATCH_OPTS_ARRAY="${package_var_name}_PATCH_OPTS_ARRAY[@]"
 
-  for patch in "${patches_path}/${package_name}/"*; do
+  local patch_path="${!PACKAGE_PATCH_PATH:-${PYTHON_BUILD_PATCH_PATH}/${package_name}}"
+  # path may be given in relative from working directory
+  [ -e "${patch_path}" ] || patch_path="${CWD}/${patch_path}"
+
+  local patch
+  for patch in "${patch_path}"/*; do
     if [ -f "$patch" ]; then
-      {
-        echo "Applying ${patch##*/} to ${package_name}..."
-        patch "-p${!PACKAGE_PATCH_STRIP:-0}" < "$patch"
+      echo "Applying ${patch##*/} to ${package_name}..." >&2
+      { ${!PACKAGE_PATCH:-patch} "-p${!PACKAGE_PATCH_STRIP:-0}" $PATCH_OPTS ${!PACKAGE_PATCH_OPTS} "${!PACKAGE_PATCH_OPTS_ARRAY}" < "$patch"
       } >&4 2>&1
     fi
   done
@@ -1253,6 +1260,12 @@ elif [ ! -e "$DEFINITION_PATH" ]; then
   fi
 fi
 
+if [ -z "$PYTHON_BUILD_PATCH_PATH" ]; then
+  PYTHON_BUILD_PATCH_PATH="${DEFINITION_PATH%/*}/patches/${DEFINITION_PATH##*/}"
+else
+  PYTHON_BUILD_PATCH_PATH="${PYTHON_BUILD_PATCH_PATH%/}"
+fi
+
 PREFIX_PATH="${ARGUMENTS[1]}"
 if [ -z "$PREFIX_PATH" ]; then
   usage