1. A computer system for generating custom template-based documents, the computer system comprising: a template storage device; and one or more hardware processors programmed, via executable code instructions, to implement: a template generator configured to: access a placeholder template comprising one or more placeholders indicating locations for insertion of executable instructions; receive executable instructions to be included in the placeholder template; and store, in the template storage device, a template including one or more sets of the received executable instructions inserted into corresponding placeholders of the placeholder template; a user interface unit configured to: generate one or more user interfaces configured to display one or more selectable graphics each corresponding to one or more respective data objects; and receive, via the one or more user interfaces, selection of one or more data objects to include in a custom document based at least in part on a selection of the one or more selectable graphics by a user, at least one of the selected one or more data objects comprising a person data object; a template selection unit configured to receive a selection of the template; and a template processor configured to: parse the one or more sets of executable instructions included in the template; execute each of the one or more sets of executable instructions, wherein at least one set of first executable instructions includes instructions to access properties of the selected data objects stored in one or more data sources, and wherein at least one of the accessed properties is associated with the person data object, and wherein at least one set of second executable instructions is configured to determine an additional data object and at least one property associated with the additional data object based on at least one property of the selected one or more data objects; generate an output for each of the one or more sets of executable instructions; and generate the custom document by replacing the one or more sets of executable instructions in the template with the output generated by execution of corresponding sets of executable instructions, at least some of the output including properties of the selected one or more data objects, the at least one property associated with the person data object, and the at least one property associated with the additional data object.