Asset trigger extension and wildcards

[guybedford]

From the bug report at https://spectrum.chat/zeit/now/reading-template-assets-on-file-system-using-now-node~56f167d9-7416-444f-a352-6738b341df48, we should be able to support the following pattern:

const template_location = process.cwd() + '/api/email-templates/' + template_name + name + '.html';

That is:

1. Supporting process.cwd() as a trigger for expression analysis
2. Supporting wildcards in the analysis
3. Supporting wildcards in the emission process

[styfle]

Yes please! 💚

[guybedford]

process.cwd() is actually quite tricky because we don't what it is!

What we can do here is permit process.cwd() as a build option, then in ncc itself pass the build process.cwd() to this. It still seems a little fragile, but likely the best we can do.

[guybedford]

These features are now all implemented in #30.