Debug mode, check if element is found and correct comments

[danilopolani]

According to #7, with this merge you will be able to:

1. Check if the element is found with the Error field in the Root struct;
2. Toggle debug mode with the SetDebug() function. Default is false, if set to true will show the various panic().

Example to check if the node is found (no panic will appear in the terminal):

source := soup.HTMLParse(resp)
articles := source.Find("section", "class", "loop").FindAll("article")
for _, article := range articles {
    link := article.Find("h2").Find("a")
    if link.Error == nil { // link is an instance of Root
        fmt.Println(link.Text(), "| Link :", link.Attrs()["href"])
    }
}

Example to check if the node is found with debug mode (panic will appear in terminal):

soup.SetDebug(true)

source := soup.HTMLParse(resp)
articles := source.Find("section", "class", "loop").FindAll("article")
for _, article := range articles {
    link := article.Find("h2").Find("a")
    if link.Error == nil { // link is an instance of Root
        fmt.Println(link.Text(), "| Link :", link.Attrs()["href"])
    }
}

Notes:

• I added correct comments to each function, interface and struct. Node, Root, FindNextSibling and FindPrevSibling needs edit on their comments.
• The example codes should be updated.

[anaskhan96]

Thank you for your PR!

SetDebug() function seems like a good addition to the package. Found as a field of Root however, kind of deviates from what we were discussing in #7 . Instead, an Error field returning an error (if occurred, nil otherwise), seems to lean more, if not completely, towards idiomatic Go. For example:

link := doc.Find("h2").Find("a")
if link.Error != nil {
    // handle the error
}

Node, Root, FindNextSibling, and FindPrevSibling do need proper comments for documentation which I'll be looking into.

[danilopolani]

Yes, I thought about Error which is more Goish, but Found is more logic-driven (element found or not) as boolean, anyway tomorrow I'll change it to the error type.

After this PR, I have an idea about the other open issue, let's see if it works!

[danilopolani]

Ok, I replaced Found with Error and I updated the PR description. Let me know!

[anaskhan96]

Looks good. Merged!