marked: Use startsWith instead of indexOf.
marked: Use startsWith instead of indexOf.

We are sweeping the codebase to use startsWith
when possible.  I kept this on a separate
commit due to us vendoring the library, just
to reduce some noise.

kou/zulip,synicalsyntax/zulip,showell/zulip,rht/zulip,zulip/zulip,rht/zulip,eeshangarg/zulip,andersk/zulip,eeshangarg/zulip,kou/zulip,hackerkid/zulip,brainwane/zulip,punchagan/zulip,timabbott/zulip,punchagan/zulip,synicalsyntax/zulip,andersk/zulip,brainwane/zulip,hackerkid/zulip,andersk/zulip,kou/zulip,showell/zulip,timabbott/zulip,timabbott/zulip,timabbott/zulip,timabbott/zulip,shubhamdhama/zulip,punchagan/zulip,rht/zulip,shubhamdhama/zulip,timabbott/zulip,showell/zulip,synicalsyntax/zulip,showell/zulip,showell/zulip,kou/zulip,hackerkid/zulip,brainwane/zulip,brainwane/zulip,kou/zulip,eeshangarg/zulip,andersk/zulip,shubhamdhama/zulip,eeshangarg/zulip,andersk/zulip,synicalsyntax/zulip,zulip/zulip,shubhamdhama/zulip,rht/zulip,hackerkid/zulip,punchagan/zulip,shubhamdhama/zulip,eeshangarg/zulip,eeshangarg/zulip,zulip/zulip,synicalsyntax/zulip,brainwane/zulip,timabbott/zulip,eeshangarg/zulip,zulip/zulip,hackerkid/zulip,shubhamdhama/zulip,shubhamdhama/zulip,hackerkid/zulip,synicalsyntax/zulip,punchagan/zulip,zulip/zulip,hackerkid/zulip,rht/zulip,kou/zulip,andersk/zulip,punchagan/zulip,showell/zulip,rht/zulip,zulip/zulip,brainwane/zulip,kou/zulip,showell/zulip,brainwane/zulip,rht/zulip,punchagan/zulip,zulip/zulip,synicalsyntax/zulip,andersk/zulip