`check_zero()` causes `sankeyNetwork()` and `forceNetwork()` to fail silently on specific inputs

Hi, thanks for an awesome package!

I've found an issue where assumptions made by `check_zero()` cause some graph plotting operations to fail silently if you have a 1-indexed rather than 0-indexed dataset for `Links` . I have a fix ready and will raise a PR for review in a sec.

## To reproduce

It's a bit involved so bear with me. I initially found this when using `sankeyNetwork()` but it also affects `forceNetwork()`.

First we pretend we have some 1-indexed link data:

``` r
URL <- paste0('https://cdn.rawgit.com/christophergandrud/networkD3/',
'master/JSONdata/energy.json')

energy <- jsonlite::fromJSON(URL)

# Simulate 1-indexed data
bad_energy <- energy
bad_energy$links$source <- energy$links$source + 1
bad_energy$links$source <- energy$links$source + 1
```
Normally we get a warning for this

``` r
sankeyNetwork(
Links = bad_energy$links,
Nodes = bad_energy$nodes,
Source = 'source',
Target = 'target',
Value = 'value'
)
#> Warning message:
#> It looks like Source/Target is not zero-indexed. This is required in JavaScript and so your plot may not render.
```

However, if we pass in a `tbl_df` we get no warning and no plot. We _do_ see a message about data frame conversion but nothing about the 1-indexing error, and no plot is rendered.

``` r
sankeyNetwork(
Links = tibble::as_tibble(bad_energy$links),
Nodes = bad_energy$nodes,
Source = 'source',
Target = 'target',
Value = 'value'
)
#> Links is a tbl_df. Converting to a plain data frame.
```

This is caused by the way `Links` is subset in the call to `check_zero()`:

``` r
# R/sankeyNetwork.R:82
check_zero(Links[, Source], Links[, Target])
```

The problem arises when `Links` is a `tbl_df` because `[.tbl_df` returns a `tbl_df` even if only one column is selected, unlike `[.data.frame`. This breaks `check_zero()` because it assumes its inputs are vectors.

We can get the same effect by omitting `Source` or `Target` - when these are missing the subsetting operation returns the entire data frame, which again breaks `check_zero()`.

``` r
# Both of these calls produce no plot and no warning

sankeyNetwork(
Links = bad_energy$links,
Nodes = bad_energy$nodes,
# Source = 'source',
Target = 'target',
Value = 'value'
)

sankeyNetwork(
Links = bad_energy$links,
Nodes = bad_energy$nodes,
Source = 'source',
# Target = 'target',
Value = 'value'
)
```
## Fix

I've created a PR to fix this (basically just move `check_zero()` after input checking and `tbl_df_strip()`) which I will link in a sec.

Hope this is helpful, thanks again!