Change tundra.decimal:add to include a singleton input decimal to
make it easier to use in a transformer with a hard-coded value;
closes #93
Change tundra.integer:add to include a singleton input integer
to make it easier to use in a transformer when adding a hard-coded
value; closes #94
Change tundra.integer:multiply to include a singleton input
integer to make it easier to use in a transformer when
multiplying by a hard-coded value; closes #95
Fix tundra.document:stringify to not return a null value when no
input $document is specified
Add tundra.document:blankify for converting all null values in an
IData document to empty strings; closes #92