Add support for using a YAML title field

[taylorhadden]

Original request from @jchalifour in #104:

Could cards display the YAML title field, optionally, instead of the file name as their main heading? Zettlr, for example, has a technique for this with a settings option to choose preference between filename, H1, or YAML title. Since we can already see the filename in the hierarchy, having the YAML title as a main source in the cards lens could be a useful thing.