Load documents referenced in source() calls

[renkun-ken]

Closes #20

This PR detects top-level source() calls and load documents referenced in such calls.

For example,

sources.R:

source("source1.R")

source1.R:

source("source2.R")

f <- function(x) {
  x + 1
}

source2.R:

g <- function(x) {
  x + 1
}

Opening sources.R will also load source1.R and source2.R.

[randy3k]

Didn't check the code yet. Does it handle

p <- "path"
source(p)

[renkun-ken]

Closing as the design is still problematic.