Make search case-insensitive

[susisu]

I found there are two problems about case-sensitivity of file system.

• On case-sensitive file system, it is sometimes inconvenient that gibo cannot find a .gitignore if the case does not match.
  For example, gibo macOS is OK but gibo MacOS is not.
• On case-insensitive file system, both gibo macOS and gibo MacOS works, but the URL added to the first line is wrong if the case does not match.
  If I execute gibo MacOS I get a URL https://raw.github.com/github/gitignore/982268df04842c9b34b7bdb0c4b0696b7db6e6fd/Global/MacOS.gitignore, it's 404.

This patch fixes both problems by doing case-insensitive search and using actual filenames to make URLs.

Perhaps the Windows version should also be changed, but I don't know much about Windows, need help.

[simonwhitaker]

Oh, nice catch! Thanks for the fix. 👍