Add render flag to parse local markdown files

[MasterOdin]

Adds a render option (-r or --render) which assumes that all commands are then paths to local files and renders them for the user (for testing purposes).

[felixonmars]

The flag looks a bit ambiguous to me. I guess it would make more sense to specify a remote to override the default one? (And support something like file:///)

[MasterOdin]

This is the same flag as in both the tldr-node-client and tldr-cpp-client and so is consistent in that regard.

Additionally, it strikes me that --source file:///path/to/files seems more unintuitive as you're then special casing the file:/// source to not check for platform, go through cache, etc.

[felixonmars]

Indeed. Thanks for the patch!