Added methods setSource to Block, Cell and ListItem

[robertpanzer]

This PR tackles points 2 & 3 of #513

It adds the method setSource(String) to Block, ListItem and Cell.
There's also a new method Block.setLines(List<String>) to set the source lines.

[jiangrij]

Hi robertpanzer,
I tried to use setSource(String) to a Cell, but it does not work for generating HTML(the cell source was not updated). Is there anything else I missed? Could you please advise?

The following is the code which extends from Treeprocessor:
----------------- code part ---------------------
public Document process(Document document) {
List blockslist1 = document.getBlocks();
.......
List blockslist = section.getBlocks();
for (StructuralNode abstractBlock : blockslist) {
if ((abstractBlock instanceof Table)) {
Table table = (Table) abstractBlock;
for (Row row : table.getBody()) {
List cells = row.getCells();
for (int cellindex = 0; cellindex < cells.size(); cellindex++) {
Cell cell = cells.get(cellindex);
cell.setSource("New Source");

Thanks.

[robertpanzer]

It looks like we're missing a method setText() at the moment.
For now you will have to create a new Cell with Processor.createTableCell(column, "New Source") and replace it in the cells.

[robertpanzer]

I just tried an example like this and it worked for me.
Are you sure that the your conditions found the table?

[robertpanzer]

@jiangrij I just added a test in #638 that shows that updating the source of a cell works.