Adding datasourceExists function

[hairyhenderson]

Fixes #93

This does 2 things:

1. Teaches datasource to be a bit more friendly when an undefined datasource is referenced:

$ echo '{{datasource "test"}}' | gomplate
2017/02/02 21:09:46 Undefined datasource 'test'

2. Adds a new datasourceExists function to test whether or not a given datasource was defined:

$ echo '{{if (datasourceExists "test")}}{{datasource "test"}}{{else}}no worries{{end}}' | gomplate
no worries

Signed-off-by: Dave Henderson dhenderson@gmail.com

[hairyhenderson]

@bartmeuris what do you think of this as a fix for #93?

[bartmeuris]

Looks good, pretty much what I had in mind. Sorry I couldn't submit a PR myself, currently a bit time-constrained. Code looks pretty clean and straight-forward :)

[hairyhenderson]

@bartmeuris 👍