Add main property to package.json

[ctaepper]

having the main property follows npm best practices for simpler including the main file of the package.

for example, when using cssloader in webpack

/* before */
.container {
  composes: db from 'tachyons-display/src/tachyons-display.css';
}

/* after */
.container {
  composes: db from 'tachyons-display';
}

i'll try to do PRs for all modules, any help is appreciated... otherwise, please try to let me know rather sooner that later, if that work would be wasted, because you don't want that feature :)

[ctaepper]

PS: i'm not gonna add any PR descriptions to the other modules, i assume once is enough

[ctaepper]

forgive me for being blind, there is already the style property for this path. however i'm not sure i can webpack make evaluate this porperty? maybe @sokra could shed some light?

[ctaepper]

apparently its talked about here: webpack/css-loader#51