add an option -bs-main

[bobzhang]

The idea is for small to medium projects, user should not bother to write a build system, it just works.

bsc -bs-main main.ml

It will build its module and all its dependencies.

TODO:

• run node

bsc -c -bs-main main.ml (* compile only *)
bsc -bs-main main.ml (* compile and run the command `node main.js` directly)

• performance tuning, parallelize building, cache files

[coveralls]

Coverage remained the same at 50.731% when pulling 72338b9 on bs-main into 54d4571 on master.

[coveralls]

Coverage remained the same at 50.731% when pulling 90f5836 on bs-main into a4aaf4c on master.

[coveralls]

Coverage remained the same at 50.731% when pulling 4293d63 on bs-main into a4aaf4c on master.

[coveralls]

Coverage remained the same at 50.731% when pulling 6591077 on bs-main into a4aaf4c on master.