add source_dir argument to using_remote_theme()

[zkamvar]

This will fix #576

[zkamvar]

I've tested this on the lesson-example repo and it works (on my machine)

$ make lesson-check-all
./CONTRIBUTING.md: Line(s) too long: 3, 114, 126, 131, 135
./README.md: Line(s) too long: 1
./_episodes/02-tooling.md:125: Unknown or missing code block type html
./_episodes/05-rmarkdown-example.md: Line(s) too long: 146, 163
./_episodes/05-rmarkdown-example.md:68: Unknown or missing code block type None
./_episodes/05-rmarkdown-example.md:98: Unknown or missing code block type None
./_episodes/05-rmarkdown-example.md:121: Unknown or missing code block type None
./_episodes/05-rmarkdown-example.md:145: Unknown or missing code block type None
./_episodes/05-rmarkdown-example.md:179: Unknown or missing code block type None
./_episodes/06-style-guide.md: Line(s) too long: 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 150, 151, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161, 162, 163, 164, 165, 166, 201
./_episodes/06-style-guide.md:109: Unknown or missing code block type html
./_episodes/06-style-guide.md:174: Unknown or missing code block type html
./_episodes/06-style-guide.md:199: Unknown or missing blockquote type None
./_episodes/06-style-guide.md:204: Unknown or missing blockquote type None
./setup.md:182: Unknown or missing code block type None
./setup.md:452: Unknown or missing blockquote type None

[maxim-belkin]

Thanks for the fix, Zhian! I'm puzzled as to how I missed this in my tests 😕

[zkamvar]

It's alright and completely understandable!