add build-related files and dirs to .npmignore file

[Pomax]

The current release includes a bunch of things that are only relevant for the build process, rather than being part of the code when used as a dependency in a project. Some things that might be good to add to the npmignore file:

• the .github directory
• the build directory
• the webpack config file
• the Travis config file
• The Bower config file

(In an ideal world, the dist folder would also be ignored with a separate @rss-parser/browser dependency that folks can use if they need a browser version, but that's far more work to set up so a bit unreasonable to ask for =)