Enable to parse local sitemap regardless of filename

[tsmsogn]

This PR has following features.

• Enable to parse local sitemap regardless of file name
• Fixes Local sitemap with relative path doesn't work #14

[benbalter]

Based on the error in #14, I wonder if we need to call File.expan_path on @url such that the URL is prefexed by a / in all cases.

[tsmsogn]

Thank you for your reply.

Should I close this PR, then split this PR into two PRs?
One fixes #14 with File.expan_path, and the other enables to parse sitemap regardless of file name.

[benbalter]

and the other enables to parse sitemap regardless of filename

Can you explain a bit more about why you need to do that?

[tsmsogn]

Can you explain a bit more about why you need to do that?

There are two reasons.

1. I think sitemap can have various file name. For example, It is used in sitemap index.
2. I'd like to parse sitemap in local regardless of file name.